1.先备份一下源文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2.删除掉/etc/apt/sources.list中的内容
sudo vi /etc/apt/sources.list
并将一下内容粘贴进去并保存
阿里源
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
下面是Debian默认源
deb http://deb.debian.org/debian bullseye main contrib non-free
deb-src http://deb.debian.org/debian bullseye main contrib non-free
deb http://deb.debian.org/debian-security/ bullseye-security main contrib non-free
deb-src http://deb.debian.org/debian-security/ bullseye-security main contrib non-free
deb http://deb.debian.org/debian bullseye-updates main contrib non-free
deb-src http://deb.debian.org/debian bullseye-updates main contrib non-free
3.执行下列命令测试一下
sudo apt update
4.如果发生了关于signature的问题，运行下面命令
apt-key adv --keyserver keyserver.ubuntu.com --recv {your pub key}
