默认的官方 apt-get 源，在国内特别慢，执行 apt-get update 指令时几乎没反应，而替换为阿里云的 debian 源可以大大加快速度。

先备份 /etc/apt/sources.list ，然后替换 /etc/apt/sources.list 为下面的内容：

deb http://mirrors.aliyun.com/debian stable main contrib non-free
deb http://mirrors.aliyun.com/debian stable-proposed-updates main contrib non-free
deb http://mirrors.aliyun.com/debian stable-updates main contrib non-free
deb-src http://mirrors.aliyun.com/debian stable main contrib non-free
deb-src http://mirrors.aliyun.com/debian stable-proposed-updates main contrib non-free
deb-src http://mirrors.aliyun.com/debian stable-updates main contrib non-free