目前Ubutun版本更新迭代太快～常规版本的维护期都比较短，基本一年以内，只有像10.04,12.04,14.04还有以后的16.04才会长久支持3-5年。
1. 首先备份一下默认源(养成习惯)
cp /etc/apt/sources.list /etc/apt/sources.list.default2. 修改/etc/apt/sources.list
vi /etc/apt/sources.list把所有源的地址换成 http://old-releases.ubuntu.com/ubuntu ，注意不要把后面的系统版本和分类删了。
deb http://old-releases.ubuntu.com/ubuntu vivid main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-security main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-proposed main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-backports main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-security main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-updates main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-proposed main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-backports main restricted universe multiverse更新源列表：
apt-get update