推荐中科大的源，稳定又快~
大致步骤- 备份/etc/apt/sources.list
- 修改/etc/apt/sources.list的内容为以下内容，保存
- sudo apt-get update
- sudo aot-get upgrade
文件内容：(中科大)
deb https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse deb https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse deb-src https://mirrors.ustc.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
更多源：
Ubuntu 20.04换国内源 清华源 阿里源 中科大源 163源_xiangxianghehe的博客-CSDN博客https://blog.csdn.net/xiangxianghehe/article/details/105688062
