查看本系统的系统代号

lsb_release -c

Ubuntu 20.04 LTS 代号为focal。
Ubuntu 18.04 LTS 代号为bionic。

Desktop 版

0.0 打开设置：

0.1 手动选择阿里源（系统自动选择中国站点可能会很慢）

0.2 重新载入软件包

终端操作 - 适用于 Server & Desktop 版本

1.0 首先我们需要备份原来的软件源文件（防止改错），然后删除掉 sources.list （因为我们要直接添加内容）

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

1.1 使用编辑器修改sources.list内容（这里使用vim、阿里源）

vim /etc/apt/sources.list
删除sources.list所有内容，替换以下内容

针对 Ubuntu 20.04 LTS
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

针对 Ubuntu 18.04 LTS
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

1.2 刷新软件源信息

sudo  apt update
sudo  apt upgrade