更换国内源目的：
使用Ubuntu U盘进行安装，使用terminal进行文件源下载各种问题以及网络速度慢等问题，考虑更换国内源进行支持。 目前 阿里源 清华源 网易源 为流行。
先备份原文档：
mv /etc/apt/sources.list /etc/apt/sourses.list.backup
移动到对应sourses.list文档目录下使用 chomed更改只读权限：
cd /etc/apt
sudo chmod 777 sourses.list
打开文档，删除内容，更换源地址
sudo gedit /etc/apt/sources.list
阿里源：
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb 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 main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
更换成功后保存退出，更新
sudo apt-get update
即可完成
