装好ubuntu系统后的第一见事就是替换自带的更新源，原因是系统自带的源有些在中国访问不了，可以访问的速度又特别慢。
备份sources.list文件：
cp /etc/apt/sources.list /etc/apt/sources.list.backup
打开sources.list： (打开之前最好是安装vim，vi确实不太好用 apt-get install vim)
vi /etc/apt/sources.list
修改sources.list，将sources.list内容全部替换为以下内容：（个人觉得sohu的源比较快吧）
deb http://mirrors.sohu.com/ubuntu/ precise main restricted
deb-src http://mirrors.sohu.com/ubuntu/ precise main restricted
deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted
deb http://mirrors.sohu.com/ubuntu/ precise universe
deb-src http://mirrors.sohu.com/ubuntu/ precise universe
deb http://mirrors.sohu.com/ubuntu/ precise-updates universe
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates universe
deb http://mirrors.sohu.com/ubuntu/ precise multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-updates multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-security main restricted
deb-src http://mirrors.sohu.com/ubuntu/ precise-security main restricted
deb http://mirrors.sohu.com/ubuntu/ precise-security universe
deb-src http://mirrors.sohu.com/ubuntu/ precise-security universe
deb http://mirrors.sohu.com/ubuntu/ precise-security multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-security multiverse
保存对source.list的修改。
更新源信息：
sudo apt-get update
Ubuntu 15.10更新源已可用，更新到Ubuntu 15.10的方法：http://www.linuxdiyf.com/linux/11955.html
Ubuntu 15.04更新源：http://www.linuxdiyf.com/linux/11839.html
为Linux配置apt-get的更新源：http://www.linuxdiyf.com/linux/12073.html