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