1.备份 cp /etc/apt/sources.list /etc/apt/sources.list.bak
2.打开配置文件 sudo gedit /etc/apt/sources.list
3.中科大(推荐)
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
4.手动更新
sudo apt update
sudo apt upgrade
update 和upgrade的区别
update, 会访问源列表里的每个网址，并读取软件列表，然后保存在本地,软件包管理器里看到的软件列表，都是通过update命令更新的
upgrade 把本地已安装的软件，与软件列表里对应软件进行对比，如果发现已安装的软件版本太低，就会提示你更新
总结：update就是检查，看看自己安装的软件有没有新版本，但不进行更新，upgrade才进行真正的软件更新。