修改 /etc/apt/sources.list 文件，也即修改镜像源，能够加快在 Ubuntu 中下载和更新相关软件数据；否则默认情况下使用的是外网，下载起来比较慢！！！

基本步骤：

1、复制一份 /etc/apt/sources.list 文件，以作备用,其中 sources-bak.list 是备份文件名；

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

2、打开  /etc/apt/sources.list  文件；

 sudo vi /etc/apt/sources.list

3、点击 D 删除里面的所有内容；

4、打开镜像源 阿里云镜像源链接 ；

5、找到 Ubuntu 对应版本的镜像； 

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

 6、将其内容复制到 /etc/apt/sources.list 文件下；

7、点击 ESC 然后输入“：wq”，保存并退出；

8、更新镜像源；

sudo apt update
sudo apt upgrade