目的：替换掉Ubuntu自带的源

Ubuntu自带的源 cn.archive.ubuntu.com速度并不快，用中科大源替换。

步骤：

1.备份源文件

  1. cd /etc/apt
  2. cp sources.list sources.list.bak

2.写入中科大源

中科大源地址
在最后的ubuntu源里选择20.04，获得到如下内容。

  1. deb https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
  2. deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
  4. deb https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
  5. deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
  7. deb https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
  8. deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
  10. deb https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
  11. deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
  13. ## Not recommended
  14. # deb https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
  15. # deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse

清空原 sources.list 内容，用上面的内容替换。
最后执行：

  1. sudo apt update