ubuntu更换国内源

1. 打开终端，输入以下命令，备份原有的sources.list文件：

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

2. 打开sources.list文件：

   sudo gedit /etc/apt/sources.list

3. 将原有的源地址全部注释掉，添加以下内容（以阿里云源为例）：

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

注意：上面的源地址适用于Ubuntu 20.04版本，如果你的版本不同，请复制对应版本的源地址。

4. 保存文件并退出gedit。

5. 更新软件包信息：

   sudo apt-get update

6. 更新软件包：

   sudo apt-get upgrade

7. 完成，现在你已经更换了国内源。