1.备份原来的sources.list。

源文件。

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

2.修改sources.list文件内容。

sudo nano /etc/apt/sources.list

按CTRL+K删除所有行。

然后粘贴阿里云mirrors官网的对应Ubuntu版本的镜像地址列表

我的版本是Ubuntu 20.04，镜像地址列表是：

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

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

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

按CTRL+O保存，CTRL+X退出nano。

3.更新源和更新软件

sudo apt-get update #更新源  
sudo apt-get upgrade #更新软件 

搞定。