网上应该可以找到很多关于ubuntu源的设置方法，但是如果不搞清楚就随便设置的话，不仅不能起到应有的效果，还会由于一些问题导致apt不可用。
最正确的更换源的方法应该如系统提示的：
## a.) add 'apt_preserve_sources_list: true' to /etc/cloud/cloud.cfg
## or do the same in user-data
## b.) add sources in /etc/apt/sources.list.d
## c.) make changes to template file /etc/cloud/templates/sources.list.tmpl
这种方法有点没弄明白在/etc/apt/sources.list.d应该添加的是什么内容，如果是源文件的话，最后更改的模板又是什么作用？也没有去尝试，有时间会解决一下。
以下提供设置阿里源的方法，其他源也可以如法炮制：
1. 首先查看自己的ubuntu系统的codename，这一步很重要，直接导致你更新的源是否对你的系统起效果，查看方法：
lsb_release -a
如，我的系统显示：
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.2 LTS
Release: 14.04
Codename: trusty
显示了一些ubuntu的版本信息，需要得到的是Codename，比如，我这里是trusty
2. 确认阿里源支持：
登陆以下网页：http://mirrors.aliyun.com/ubuntu/dists/
该网页显示了阿里云支持的ubuntu系统下各个Codename版本，确保自己的Codename在该网页中存在（一般都会有的）
2. 备份系统源：
cd /etc/apt
sudo mv sources.list sources.list_bak
3. 添加新的源文件：
sudo vi sources.list
并添加以下内容：注意，每一行的trusty应该用第一步查看得到的Codename来代替
# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial main restricted #Added by software-properties
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted
deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted multiverse universe #Added by software-properties
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted multiverse universe #Added by software-properties
deb http://mirrors.aliyun.com/ubuntu/ xenial universe
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates universe
deb http://mirrors.aliyun.com/ubuntu/ xenial multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse #Added by software-properties
deb http://archive.canonical.com/ubuntu xenial partner
deb-src http://archive.canonical.com/ubuntu xenial partner
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted multiverse universe #Added by software-properties
deb http://mirrors.aliyun.com/ubuntu/ xenial-security universe
deb http://mirrors.aliyun.com/ubuntu/ xenial-security multiverse
4. 保存并且sudo apt-get update，更新成功
5. 以下提供配置脚本：
1 Codename=$( (lsb_release -a)|awk '{print $2}'|tail -n 1 )
2 echo "
3 deb http://mirrors.aliyun.com/ubuntu/ $Codename main multiverse restricted universe
4 deb http://mirrors.aliyun.com/ubuntu/ $Codename-backports main multiverse restricted universe
5 deb http://mirrors.aliyun.com/ubuntu/ $Codename-proposed main multiverse restricted universe
6 deb http://mirrors.aliyun.com/ubuntu/ $Codename-security main multiverse restricted universe
7 deb http://mirrors.aliyun.com/ubuntu/ $Codename-updates main multiverse restricted universe
8 deb-src http://mirrors.aliyun.com/ubuntu/ $Codename main multiverse restricted universe
9 deb-src http://mirrors.aliyun.com/ubuntu/ $Codename-backports main multiverse restricted universe
10 deb-src http://mirrors.aliyun.com/ubuntu/ $Codename-proposed main multiverse restricted universe
11 deb-src http://mirrors.aliyun.com/ubuntu/ $Codename-security main multiverse restricted universe
12 deb-src http://mirrors.aliyun.com/ubuntu/ $Codename-updates main multiverse restricted universe ">sources.list
13 apt-get update
sudo运行该脚本即可（注意运行之前最好备份之前的sources.list）