1.备份原系统中的镜像源
sudo cp /etc/apt/sources.list /etc/apt/sources.bak1
#第一个参数时拷贝的文件路径和文件名称,第二个是拷贝到(粘贴)的文件路径和文件名
sudo cp /etc/apt/sources.list /etc/apt/sources.bak1
2.更改镜像源文件的访问权限(增加写权限)
sudo chmod u+w /etc/apt/sources.list
sudo chmod u+w /etc/apt/sources.list
更改完成可以再恢复文件的访问权限。
清华镜像源：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
3.更改镜像源内容
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