默认安装好的ubuntu是国外的源，下载和更新软件比较慢！！！ 换到国内源可以加快更新
# 备份原来的源并且另存
sudo cp -v /etc/apt/sources.list /etc/apt/sources.list.backup #
# 执行chmod命令更改文件权限使软件源文件可编辑
sudo chmod 777 /etc/apt/sources.list
# 通过gedit命令编辑软件源：
sudo gedit /etc/apt/sources.list
# 执行上面命令后如果报sudo: gedit: command not found的错误，执行下面命令
sudo apt install vim vim /etc/apt/sources.list
打开源文件后，按 i 进入编辑模式将所有的代码删干净，然后复制粘贴你需要的镜像源，先按ECS键退出编辑，然后使用 :wq 命令退出编辑模式
换完源之后执行以下命令
sudo apt update
sudo apt upgrade
可复制如下的内容粘贴到sources.list中（复制其中一个源即可）
阿里云国内源Ubuntu
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
阿里云国内源Ubuntu（包括预发布的源）
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multivers
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
#源码镜像
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
2.清华大学源Ubuntu
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
说明：
main 官方支持，自由软件。
restricted官方支持，非完全自由软件。
universe非官方支持，社区维护的自由软件。
multiverse非官方支持，非自由软件。
focal-proposed 为预发布软件源(如果为了稳定性不建议添加到更新源中)