在ubuntu中替换清华源时候遇到的问题
清华源： https://mirror.tuna.tsinghua.edu.cn/help/ubuntu/
先备份默认的官方源
sudo mv /etc/apt/sources.list /etc/apt/sources.list.bak
sudo vi /etc/apt/sources.list
把上面的链接拷贝到文件中去：
# 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/ focal main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse # 预发布软件源，不建议启用 # deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
apt-get update
发现更新失败
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-backports Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-security Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.
因为默认的源是 http 的，但是准备使用的镜像源是 https 的，所以需要额外的安装有关 HTTPS 的包
所以安装这两个包之后就可以正常拉取https的源了：
sudo apt install apt-transport-https
sudo apt install ca-certificates
