Ubuntu安装后文件/etc/apt/sources.list中自带的源为Ubuntu官方源，由于服务器在国外，访问速度很慢，所以用国内源代替。
备份
cp /etc/apt/sources.list /etc/apt/sources.list.bak （不是root用户，请用sudo授权）
修改
vim /etc/apt/sources.list 进入后可全部删除里面的内容，然后复制下面的源进行替换即可
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
最后保存退出，回到命令行后执行更新命令
：wq apt-get update ------ 更新源 apt-get upgrade ------ 升级软件，会把系统装的软件升级到最新，时间比较长，有需要的可以选择执行。
也可以访问阿里的开源镜像站https://developer.aliyun.com/mirror/，自己去复制对应版本的的源。
