首先备份本地的源
cp /etc/apt/sources.list /etc/apt/sources.list.bak编辑源文件
vi /etc/apt/sources.list删除原有内容，添加下面内容
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
更新源
sudo apt-get update
sudo apt-get upgrade