Ubuntu 18.04如何更换源

1.输入该命令按回车键拷贝源文件进行备份：

`cp /etc/apt/sources.list /etc/apt/sources.list.bak`

2.输入该命令按回车键：`vi /etc/apt/sources.list`

3.按`i`进入插入模式，将文件内容修改为如下内容，然后按`ESC`键输入`wq`按回车键保存退出vi编辑 

deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse \# 
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse \# 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

4.输入该命令按回车键更新列表：`apt-get update`