系统为LTS，并且未被废弃
升级一个版本
apt-get update
apt-get dist-upgrade
升级多个版本
修改/etc/apt/source.list
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted #修改xenial为你要升级到的版本的代号
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted
执行更新
apt-get update
apt-get dist-upgrade
