要知道当前系统的代号，可以用以下命令：

lsb_release -a

2到阿里源看下这个源存在不存在

http://archive.ubuntu.com/ubuntu/dists/

然后以下面的为模板，把'TODO'的地方换成上面的Codename

deb http://mirrors.aliyun.com/ubuntu/ TODO main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ TODO main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ TODO-security main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ TODO-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ TODO-updates main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ TODO-updates main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ TODO-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ TODO-proposed main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ TODO-backports main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ TODO-backports main restricted universe multiverse

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-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed 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

然后粘贴到刚才的/etc/apt/sources.list里。

3 更新缓存和升级

sudo apt-get update

sudo apt-get upgrade