1）sudo cp  /etc/apt/sources.list    /etc/apt/sources.list_backup

（2）sudo gedit /etc/apt/sources.list

将下面的模板内容，复制粘贴到 /etc/apt/sources.list 文件里面后，保存就可以了：（我们使用的Ubuntu 是16.04）

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial universe

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates universe

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial multiverse

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates multiverse

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security universe 

deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security multiverse

deb http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
##测试版源
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
# 源码
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
##测试版源
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse

现在，再执行：

sudo apt-get update

Linux Ubuntu config source