Ubuntu安装更新失败
好久没有用Ubuntu登录了，今天用登录进入，提示说，有更新要安装，估计也有大半年了，那就更吧。
结果提示说“请确认网络是否连接“，总之更是失败，提示说”NOT FOUND xxxx“，应该是连接不到服务器。
404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/main libcurl3 i386 7.22.0-3ubuntu4.7 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/main libcurl3-nss i386 7.22.0-3ubuntu4.7 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/main libwbclient0 i386 2:3.6.3-2ubuntu2.9 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/main libyaml-dev i386 0.1.4-2ubuntu0.12.04.2 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/main libyaml-0-2 i386 0.1.4-2ubuntu0.12.04.2 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/universe openjdk-7-jdk i386 7u51-2.4.4-0ubuntu0.12.04.2 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/universe openjdk-7-jre i386 7u51-2.4.4-0ubuntu0.12.04.2 404 Not Found [IP: 91.189.92.201 80] 错误 http://security.ubuntu.com/ubuntu/ precise-security/universe icedtea-7-jre-jamvm i386 7u51-2.4.4-0ubuntu0.12.04.2
百度了一下，找到了解决办法。
原文地址：http://www.cnblogs.com/eastson/archive/2012/08/24/2654163.html
1. 首先更新源列表
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 （打开Ubuntu 12.04源列表文件）
3. 将打开的TXT，修改为国内的源
163源
deb http://mirrors.163.com/ubuntu/ precise main restricted deb-src http://mirrors.163.com/ubuntu/ precise main restricted deb http://mirrors.163.com/ubuntu/ precise-updates main restricted deb-src http://mirrors.163.com/ubuntu/ precise-updates main restricted deb http://mirrors.163.com/ubuntu/ precise universe deb-src http://mirrors.163.com/ubuntu/ precise universe deb http://mirrors.163.com/ubuntu/ precise-updates universe deb-src http://mirrors.163.com/ubuntu/ precise-updates universe deb http://mirrors.163.com/ubuntu/ precise multiverse deb-src http://mirrors.163.com/ubuntu/ precise multiverse deb http://mirrors.163.com/ubuntu/ precise-updates multiverse deb-src http://mirrors.163.com/ubuntu/ precise-updates multiverse deb http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse deb-src http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse deb http://mirrors.163.com/ubuntu/ precise-security main restricted deb-src http://mirrors.163.com/ubuntu/ precise-security main restricted deb http://mirrors.163.com/ubuntu/ precise-security universe deb-src http://mirrors.163.com/ubuntu/ precise-security universe deb http://mirrors.163.com/ubuntu/ precise-security multiverse deb-src http://mirrors.163.com/ubuntu/ precise-security multiverse deb http://extras.ubuntu.com/ubuntu precise main deb-src http://extras.ubuntu.com/ubuntu precise main
sohu源
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 deb http://extras.ubuntu.com/ubuntu precise main deb-src http://extras.ubuntu.com/ubuntu precise main
ubuntu.cn99.com源（推荐）:
deb http://ubuntu.cn99.com/ubuntu/ precise main restricted universe multiverse deb http://ubuntu.cn99.com/ubuntu/ precise-updates main restricted universe multiverse deb http://ubuntu.cn99.com/ubuntu/ precise-security main restricted universe multiverse deb http://ubuntu.cn99.com/ubuntu/ precise-backports main restricted universe multiverse deb http://ubuntu.cn99.com/ubuntu-cn/ precise main restricted universe multiverse
4. 更新源
sudo apt-get update
好了，这样刷新之后，就可以更新了。