更改更新源为 16.04 (xenial)
方法一：修改初始的更新源文件
备份初始的源文件为sources.list.ORIG，将sources.list里的"trusty"全替换为"xenial"。$ sed -i.ORIG 's/trusty/xenial/g' /etc/apt/sources.list
# 方法二（推荐）：把更新源直接改为国内的阿里云Ubuntu(xenial)镜像，这样会很快
编辑更新源文件
$ vim /etc/apt/sources.list
备份源文件后粘贴如以下代码，并保存退出。
deb http://mirrors.aliyun/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.aliyun/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.aliyun/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.aliyun/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.aliyun/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.aliyun/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.aliyun/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.aliyun/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.aliyun/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun/ubuntu/ xenial-backports main restricted universe multiverse
如果/etc/apt/sources.list.d/这个文件夹不能空，需要备份下，空的话就不需要备份。
mv /etc/apt/sources.list.d/ /etc/apt/sources.list.d.back/
mkdir /etc/apt/sources.list.d/
升级至 16.04 (xenial)
aptitude update
aptitude safe-upgrade -y
注1：升级过程中会提示你重启服务（restart services），选yes
注2：还会出现文件冲突，保留当前版本（current version）即可，输入N
依次执行如下命令：
# aptitude update
# aptitude safe-upgrade -y
#apt dist-upgrade
#apt install aptitude
#apt autoremove
以上都成功后;
# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.3 LTS
Release: 16.04
Codename: xenial
之后就可以用使用网络命令了！