国内有很多Ubuntu的镜像源，比如：阿里源、网易源等，还有很多教育网的源，比如：清华源、中科大源等。
这里以清华源为例讲解如何修改Ubuntu 18.04里面默认的源。
修改步骤
第一步：备份原始源文件，当然需要系统管理员权限操作
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup
第二步：修改sources.list文件并添加国内源
vi /etc/apt/sources.list
第三步：注释原文件内的源（Ubuntu官方源）
#Ubuntu 官方源 deb http://archive.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ gutsy-security main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ gutsy-updates main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ gutsy-proposed main restricted universe multiverse deb http://archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ gutsy-security main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ gutsy-updates main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ gutsy-proposed main restricted universe multiverse deb-src http://archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
第四步：在文件中添加清华源，保存并退出
#清华源 deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
第五步：更新源
sudo apt-get update
第六步：到此完成国内源修改
其他源
阿里源：
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
网易源：
deb http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse deb http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse deb http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse deb http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse deb http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
中科大源：
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
浙大源：
deb http://mirrors.zju.edu.cn/ubuntu/ trusty main multiverse restricted universe deb http://mirrors.zju.edu.cn/ubuntu/ trusty-backports main multiverse restricted universe deb http://mirrors.zju.edu.cn/ubuntu/ trusty-proposed main multiverse restricted universe deb http://mirrors.zju.edu.cn/ubuntu/ trusty-security main multiverse restricted universe deb http://mirrors.zju.edu.cn/ubuntu/ trusty-updates main multiverse restricted universe deb-src http://mirrors.zju.edu.cn/ubuntu/ trusty main multiverse restricted universe deb-src http://mirrors.zju.edu.cn/ubuntu/ trusty-backports main multiverse restricted universe deb-src http://mirrors.zju.edu.cn/ubuntu/ trusty-proposed main multiverse restricted universe deb-src http://mirrors.zju.edu.cn/ubuntu/ trusty-security main multiverse restricted universe deb-src http://mirrors.zju.edu.cn/ubuntu/ trusty-updates main multiverse restricted universe
搜狐源：
deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted deb http://mirrors.sohu.com/ubuntu/ precise universe deb-src http://mirrors.sohu.com/ubuntu/ precise universe deb http://mirrors.sohu.com/ubuntu/ precise-updates universe deb-src http://mirrors.sohu.com/ubuntu/ precise-updates universe deb http://mirrors.sohu.com/ubuntu/ precise multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise multiverse deb http://mirrors.sohu.com/ubuntu/ precise-updates multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise-updates multiverse deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
上海交大源：
deb http://ftp.sjtu.edu.cn/ubuntu/ dapper main multiverse restricted universe deb http://ftp.sjtu.edu.cn/ubuntu/ dapper-backports main multiverse restricted universe deb http://ftp.sjtu.edu.cn/ubuntu/ dapper-proposed main multiverse restricted universe deb http://ftp.sjtu.edu.cn/ubuntu/ dapper-security main multiverse restricted universe deb http://ftp.sjtu.edu.cn/ubuntu/ dapper-updates main multiverse restricted universe deb http://ftp.sjtu.edu.cn/ubuntu-cn/ dapper main multiverse restricted universe
北京理工大源：
deb http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe
北京交大源：
deb http://mirror.bjtu.edu.cn/ubuntu/ raring main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ raring-backports main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ raring-proposed main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ raring-security main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ raring-updates main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ raring main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ raring-backports main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ raring-proposed main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ raring-security main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ raring-updates main multiverse restricted universe