預設安裝好的ubuntu是國外的源，下載和更新軟體比較慢！！！ 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命令更改檔案許可權使軟體原始檔可編輯
sudo chmod 777 /etc/apt/sources.list
# 通過gedit命令編輯軟體源：
sudo gedit /etc/apt/sources.list
# 執行上面命令後如果報sudo: gedit: command not found的錯誤，執行下面命令
sudo apt install vim vim /etc/apt/sources.list

開啟原始檔後，按 i 進入編輯模式將所有的程式碼刪乾淨，然後複製貼上你需要的映象源，先按ECS鍵退出編輯，然後使用 :wq 命令退出編輯模式
換完源之後執行以下命令
sudo apt update
sudo apt upgrade
可複製如下的內容貼上到sources.list中（複製其中一個源即可）

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（包括預釋出的源）
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 multivers
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
#原始碼映象
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
# 預設註釋了原始碼映象以提高 apt update 速度，如有需要可自行取消註釋
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 官方支援，自由軟體。
restricted官方支援，非完全自由軟體。
universe非官方支援，社群維護的自由軟體。
multiverse非官方支援，非自由軟體。
focal-proposed 為預釋出軟體源(如果為了穩定性不建議新增到更新源中)