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