Ubuntu20.04的换源和Ubuntu18.04有一些不同，不能直接在软件设置中更改（我没找到），所以用了命令行来进行更改，这里参考了
1、备份/etc/apt/sources.list 文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2、编辑/etc/apt/sources.list文件, 我用的vim，也可以用别的编辑器编辑
sudo vim /etc/apt/sources.list
3、在文件前面添加下面的条目(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-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
4、更新
sudo apt-get update
5、如果如出现依赖问题
sudo apt-get -f install