1、首先备份Ubuntu 11.10源列表 
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup （备份下当前的源列表）
2、修改更新源 
sudo gedit /etc/apt/sources.list
#台湾源 
deb http://tw.archive.ubuntu.com/ubuntu/ precise main universe restricted multiverse 
deb-src http://tw.archive.ubuntu.com/ubuntu/ precise main universe restricted multiverse 
deb http://tw.archive.ubuntu.com/ubuntu/ precise-security universe main multiverse restricted 
deb-src http://tw.archive.ubuntu.com/ubuntu/ precise-security universe main multiverse restricted 
deb http://tw.archive.ubuntu.com/ubuntu/ precise-updates universe main multiverse restricted 
deb-src http://tw.archive.ubuntu.com/ubuntu/ precise-updates universe main multiverse restricted
#网易源（速度很快）
deb http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse 
deb-src http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse 
deb http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted 
deb-src http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted 
deb http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted 
deb http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted 
deb-src http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted 
deb http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted 
deb-src http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted 
deb-src http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted
#电子科技大学 
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise main restricted universe multiverse 
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse 
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse 
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-security main restricted universe multiverse 
deb http://ubuntu.uestc.edu.cn/ubuntu/ precise-updates main restricted universe multiverse 
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise main restricted universe multiverse 
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse 
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-proposed main restricted universe multiverse 
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-security main restricted universe multiverse 
deb-src http://ubuntu.uestc.edu.cn/ubuntu/ precise-updates main restricted universe multiverse
#北京理工大学（真羡慕刘毅同学） 
deb http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe 
deb http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe 
deb http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe 
deb http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe 
deb http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe 
deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe
3、保存后就是
sudo apt-get update
sudo apt-get upgrade
经过6个月的开发周期，ubuntu 12.04 LTS (Precise Pangolin)终于发布了，这个版本是一个长期支持版本。官方正式版下载地址公布： http://releases.ubuntu.com/precise/ 完成同步的国内镜像站点（已经可以下载正式版本）： 中科大地址（同步完毕，可下载）：http://mirrors.ustc.edu.cn/ubuntu-releases/12.04/ 顺便提一下，其实unity用习惯了还是不错的