修改ubuntu的软件源，是修改 /etc/apt/sources.list 文件
ubuntu14.04可修改为如下配置：
deb http://mirrors.ustc.edu.cn/ubuntu/ trusty main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu/ trusty-security main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty-security main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse
其它版本的也可以找到帮助页找到
https://lug.ustc.edu.cn/wiki/mirrors/help/ubuntu
还有新浪的
http://mirrors.163.com/.help/ubuntu.html
文件修改完了，更新一下即可
apt-get update
apt-get upgrade