1.备份原始源文件 sources.list
桌面打开终端，执行命令：sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2.修改源文件sources.list
（1）终端执行命令：sudo chmod 777 /etc/apt/sources.list更改文件权限使其可编辑； （2）执行命令：sudo gedit /etc/apt/sources.list打开文件进行编辑； （3）删除原来的文件内容，复制下面的任意一个到其中并保存（常用的是阿里源和清华源，推荐阿里源，内容可以去博客复制）；
阿里源：
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
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