sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup （备份当前的源列表，以便日後需要时恢复）
sudo gedit /etc/apt/sources.list （修改sources.list文件中源的列表）
sources.list文件中尾部添加
#cn99.com 源更新服务器（江苏省常州市电信）：
deb http://ubuntu.cn99.com/ubuntu/ gutsy main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ gutsy-security main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ gutsy-updates main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ gutsy-proposed main restricted universe multiverse
deb http://ubuntu.cn99.com/ubuntu/ gutsy-backports main restricted universe multiverse
deb-src http://ubuntu.cn99.com/ubuntu/ gutsy main restricted universe multiverse
deb-src http://ubuntu.cn99.com/ubuntu/ gutsy-security main restricted universe multiverse
deb-src http://ubuntu.cn99.com/ubuntu/ gutsy-updates main restricted universe multiverse
deb-src http://ubuntu.cn99.com/ubuntu/ gutsy-proposed main restricted universe multiverse
deb-src http://ubuntu.cn99.com/ubuntu/ gutsy-backports main restricted universe multiverse
#Mirror.lupaworld.com 源更新服务器（浙江省杭州市双线服务器）：
deb http://mirror.lupaworld.com/ubuntu gutsy main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu gutsy-security main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu gutsy-updates main restricted universe multiverse
deb http://mirror.lupaworld.com/ubuntu gutsy-backports main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu gutsy main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu gutsy-security main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu gutsy-updates main restricted universe multiverse
deb-src http://mirror.lupaworld.com/ubuntu gutsy-backports main restricted universe multiverse
Ubuntu 官方源
deb http://archive.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ gutsy-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ gutsy-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ gutsy-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ gutsy-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ gutsy-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ gutsy-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
台湾的官方源：
deb http://tw.archive.ubuntu.com/ubuntu gutsy main restricted universe multiverse
deb http://tw.archive.ubuntu.com/ubuntu gutsy-security main restricted universe multiverse
deb http://tw.archive.ubuntu.com/ubuntu gutsy-updates main restricted universe multiverse
deb http://tw.archive.ubuntu.com/ubuntu gutsy-backports main restricted universe multiverse
deb http://tw.archive.ubuntu.com/ubuntu gutsy-proposed main restricted universe multiverse
deb-src http://tw.archive.ubuntu.com/ubuntu gutsy main restricted universe multiverse
deb-src http://tw.archive.ubuntu.com/ubuntu gutsy-security main restricted universe multiverse
deb-src http://tw.archive.ubuntu.com/ubuntu gutsy-updates main restricted universe multiverse
deb-src http://tw.archive.ubuntu.com/ubuntu gutsy-backports main restricted universe multiverse
deb-src http://tw.archive.ubuntu.com/ubuntu gutsy-proposed main restricted universe multiverse
防止非官方源中软件包不全的问题，请在sources.list文件中尾部添加下面这个官方源。
deb http://archive.ubuntu.org.cn/ubuntu-cn/ feisty main restricted universe multiverse
如果没有这个源，下面安装某些软件的时候会提示找不到，比如w32codecs
然后 保存编辑好的文件，执行以下命令更新。
sudo apt-get update #这一步是更新你的源列表，换源後必须执行
（如果更新速度非常慢请更换源 sudo gedit /etc/apt/sources.list ）
sudo apt-get dist-upgrade #这一步是更新软件
