cp /etc/apt/sources.list /etc/apt/sources.list.backup

如果提示权限不够就输入下面两行，先进入到超级用户，再备份

sudo -s
cp /etc/apt/sources.list /etc/apt/sources.list.backup

2 修改更新源　

打开sources.list (这就是存放更新源的文件)

gedit /etc/apt/sources.list

将下面所有内容复制，粘贴并覆盖sources.list文件中的所有内容　

# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
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

3 让更新源生效

sudo apt-get update

4 其他问题

*** Error in `appstreamcli': double free or corruption (fasttop): 0x000000000237b000 ***

该问题需要执行如下命令才能解决：
或将源改回来再试删除并重新更换更新源

sudo apt-get purge libappstream3

执行完后再次执行 sudo apt-get update 命令就正常了。