### 备份源
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

### 新增源
sudo vim /etc/apt/sources.list

阿里云：
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

清华：
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security multiverse