在virtualBox安装完系统，登录后提示系统可以升级：
Welcome to Ubuntu 14.04.3 LTS (GNU/Linux 3.19.0-25-generic x86_64)
* Documentation: https://help.ubuntu.com/
424 packages can be updated.
247 updates are security updates.
New release '16.04.2 LTS' available.
Run 'do-release-upgrade' to upgrade to it.
Last login: Sun Mar 19 16:57:40 2017 from 192.168.88.105
按照提示输入do-release-upgrade，一会蹦出来一个提示：
The required dependency 'apt (>= 1.0.1ubuntu2.13)' is not installed.
所以要把apt和dpkg（更新完apt后还会检查dpkg版本）升级到最新版：
$ sudo apt-get download apt dpkg
$ sudo dpkg -i *.deb
继续输入do-release-upgrade，接来下一路按y，经过1～2小时的下载和更新后，就可以享受16.04了！
