source tsinghua

在终端执行以下命令（第一条是备份现有服务器列表，第二条是使用Gedit编辑）
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
sudo gedit /etc/apt/sources.list
从下面各服务器列表内容中选择一段替换文件中的所有内容
保存编辑好的文件，执行以下命令更新。
sudo apt-get update
sudo apt-get dist-upgrade

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-security main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-updates main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-backports main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-security main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-updates main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse

deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse