修改Ubuntu的镜像源为清华镜像源

1、首先使用以下命令备份现有的镜像源：

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

2、使用以下命令打开镜像源文件：

sudo vim /etc/apt/sources.list  

3、在vim插入模式下使用以下内容替换掉原镜像源文件sources.list中的内容：

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse

4、保存并退出。

5、在ubuntu能正常上网的前提下，使用以下命令更新软件列表：

sudo apt-get update
sudo apt-get upgrade

经过以上流程，ubuntu使用的镜像源就是清华的镜像源了。