1、首先备份源列表(有备无患)
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup
2、修改更新源
sudo vim /etc/apt/sources.list
3、添加源地址
4、启用更新地址
sudo apt-get update
附上常用源：
网易源
deb http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse
deb-src http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse
deb http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted
deb-src http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted
deb http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted
deb http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted
deb-src http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted
deb http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted
deb-src http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted
deb-src http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted
cn99源:
deb http://ubuntu.cn99.com/ubuntu/ precise main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ precise-security main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ precise-backports main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu-cn/ precise main restricted universe multiverse
电子科技大学
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise main restricted universe multiverse
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-security main restricted universe multiverse
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise main restricted universe multiverse
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-security main restricted universe multiverse
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-updates main restricted universe multiverse
