- 备份源文件
sudo cp /etc/apt/sources.list /etc/apt/old_sources.list - 用vi打开sources.list文件
sudo vi /etc/apt/sources.list - 输入:%d清除文件的内容，然后输入i进入输入模式，粘贴清华源官网内容：mirror.tuna.tsinghua.edu.cn/help/ubuntu…
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
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
# 以下安全更新软件源包含了官方源与镜像站配置，如有需要可自行修改注释切换
deb http://security.ubuntu.com/ubuntu/ bionic-security main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu/ bionic-security main restricted universe multiverse
# 预发布软件源，不建议启用
# 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
（其中bionic为Ubuntu18.0.4版本代号，可根据版本不同自行替换） 最后输入:wq保存文件并退出
- 输入
sudo apt update，更新成功，表示换源成功
