首先备份下源文件

sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup

编辑文件

sudo vim /etc/apt/sources.list

主: vim 没有需要安装 sudo apt install vim 才能使用

里边的所有删除 替换成

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-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-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports 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

使其修改后的源生效

sudo apt update

切换后可能会出现 如下图的情况

不知道怎么解决 自行百度

方法二

ubuntu 20.04 LTS 更换阿里云源
第一步：先备份下原始源：
sudo cp /etc/apt/source.list /etc/apt/source.list.backup
第二步：修改文件   
sudo vim /etc/apt/source.list
清除原有的，替换以下源
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted
deb http://mirrors.aliyun.com/ubuntu/ focal universe
deb http://mirrors.aliyun.com/ubuntu/ focal-updates universe
deb http://mirrors.aliyun.com/ubuntu/ focal multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted
deb http://mirrors.aliyun.com/ubuntu/ focal-security universe
deb http://mirrors.aliyun.com/ubuntu/ focal-security multiverse
 保存下，即可执行更新
sudo apt update
sudo apt upgrade