默認安裝好的ubuntu是國外的源，下載和更新軟件比較慢！！！ 換到國內源可以加快更新
# 備份原來的源並且另存
sudo cp -v /etc/apt/sources.list /etc/apt/sources.list.backup #
# 執行chmod命令更改文件權限使軟件源文件可編輯
sudo chmod 777 /etc/apt/sources.list
# 通過gedit命令編輯軟件源：
sudo gedit /etc/apt/sources.list
# 執行上面命令后如果報sudo: gedit: command not found的錯誤，執行下面命令
sudo apt install vim vim /etc/apt/sources.list
打開源文件后，按 i 進入編輯模式將所有的代碼刪干凈，然后復制粘貼你需要的鏡像源，先按ECS鍵退出編輯，然后使用 :wq 命令退出編輯模式
換完源之后執行以下命令
sudo apt update
sudo apt upgrade
可復制如下的內容粘貼到sources.list中（復制其中一個源即可）
阿里雲國內源Ubuntu
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
阿里雲國內源Ubuntu（包括預發布的源）
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
#源碼鏡像
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
2.清華大學源Ubuntu
# 默認注釋了源碼鏡像以提高 apt update 速度，如有需要可自行取消注釋
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
說明：
main 官方支持，自由軟件。
restricted官方支持，非完全自由軟件。
universe非官方支持，社區維護的自由軟件。
multiverse非官方支持，非自由軟件。
focal-proposed 為預發布軟件源(如果為了穩定性不建議添加到更新源中)