1. 设置APT proxy，对于直接可以连外网的机器本步骤可以忽略
sudo vi /etc/apt/apt.conf.d/proxy.conf
Acquire::http::Proxy "http://user:password@proxy_server:port/";
Acquire::https::Proxy "http://user:password@proxy_server:port/";
2. 升级Ubuntu命令
sudo apt-get update
sudo apt-get upgrade -y
sudo apt-get dist-upgrade
sudo do-release-upgrade