更新源，就是更新一个source.list文件
1、备份旧的源文件
 cd /etc/apt
 sudo mv sources.list sources.list.bak
2、创建新的源文件
 vim sources.list
添加
deb http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
（1）
deb ：二进制文件
deb-src ：源码
如果是32位系统，deb改成  deb-i386

（2）各个源的介绍
http://wiki.ubuntu.org.cn/%E6%BA%90%E5%88%97%E8%A1%A8

（3）不同的版本，源的代号也不同，我用的是14.04代号是trusty 
Raring(13.04)版本
Quantal(12.10)版本
Precise(12.04)版本
Lucid(10.04) 版本
3、更新
sudo apt-get update