1 、打开终端，输入下面命令进行备份：

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

2 、编辑sources.list文件

在终端执行下面命令，打开sources.list文件进行编辑

sudo gedit /etc/apt/sources.list

打开文件后，删除原来的文件内容，复制下面内容到sources.list里并保存

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

deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src