首先将原有文件备份:
sudo mv /etc/apt/sources.list /etc/apt/sources.bak

编辑该文件:
sudo vi /etc/apt/sources.list

加入以下内容(国内源)

网易163更新服务器：

deb http://mirrors.163.com/debian/ squeeze main non-free contrib
deb http://mirrors.163.com/debian/ squeeze-proposed-updates main non-free contrib
deb-src http://mirrors.163.com/debian/ squeeze main non-free contrib
deb-src http://mirrors.163.com/debian/ squeeze-proposed-updates main non-free contrib

sohu 更新服务器：

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

保存文件后如下执行使源生效:
sudo apt-get update