1. vim /etc/apt/sources.list

    中科大源：http://mirrors.ustc.edu.cn/help/ubuntu.html
    下面的北外源好像出问题了，不知道什么时候修复

    1. # 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
    2. deb https://mirrors.bfsu.edu.cn/ubuntu/ focal main restricted universe multiverse
    3. # deb-src https://mirrors.bfsu.edu.cn/ubuntu/ focal main restricted universe multiverse
    4. deb https://mirrors.bfsu.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
    5. # deb-src https://mirrors.bfsu.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
    6. deb https://mirrors.bfsu.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
    7. # deb-src https://mirrors.bfsu.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
    8. deb https://mirrors.bfsu.edu.cn/ubuntu/ focal-security main restricted universe multiverse
    9. # deb-src https://mirrors.bfsu.edu.cn/ubuntu/ focal-security main restricted universe multiverse
    11. # 预发布软件源，不建议启用
    12. # deb https://mirrors.bfsu.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
    13. # deb-src https://mirrors.bfsu.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse