备份好原软件源后，更改软件源文件，然后升级。
一、备份原软件源文件sourcelist
cp /etc/apt/sources.list /etc/apt/sources.list.bak
二、修改软件源文件
vim /etc/apt/sources.list
改为如下源：
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
三、升级
更新系统软件包 ：apt update && apt dist-upgrade
重启：reboot
安装更新工具：apt install update-manager-core
使用ssh登录服务器升级，建议使用screen，防止SSH连接断开。
安装screen，启动screen，升级Ubuntu：
$ sudo apt-get install screen
$ screen
$ sudo do-release-upgrade -d