在Centos/RHEL系统中，配置yum仓库的文件是/etc/yum.repo.d/目录下的.repo文件，而在Ubuntu系统中则是/etc/apt/sources.list文件
配置系统文件的第一要素就是备份文件，一般系统文件都是kb级的文件，占用存储空间并不是很大，所以没有必要为了节省那一点空间而动用rm -rf命令，这也是为自己留一条后路，刚学习Linux那会儿我比较喜欢用rm -rf命令，用起来很过瘾，一旦出错就是毁天灭地，曾经有幸在一次系统调试中成功把服务器的动态链接库搞崩了，导致别人辛辛苦苦配了两三个小时的环境直接崩溃，就是因为没有习惯备份文件导致的苦果，对于自己的虚拟机也是经常搞崩，然后只能从另一台正常的虚拟机复制相关的文件才能解决，但物理机可没有那么方便。
首先切换到root用户
-
备份文件
cat /etc/apt/sources.list > /etc/apt/sources.list.bak -
覆盖/etc/apt/sources.list文件
将/etc/apt/sources.list文件内容全部替换成阿里源或清华源或中科大源，这种方法会将Ubuntu的源全部换成第三方源 -
追加到/etc/apt/sources.list文件
有些软件源可能在第三方源中没有，但你可能会用到，这时候就不能用覆盖的方式了，应该采用追加的方式，讲第三方源复制粘贴到/etc/apt/sources.list文件的末尾。 -
更新一下
sudo apt-get update
如果sudo apt-get update命令执行结果出现了错误则表示配置不成功，应该考虑更换一个教程，毕竟这个东西是会发生改变的，这时候就要将原来备份的内容还原回去，执行一下cat /etc/apt/sources.list.bak > /etc/apt/sources.list && apt-get update 命令，然后更换其他有效教程吧。
个人比较喜欢阿里源，在这里贴个阿里源的官方教程吧
Ubuntu阿里源官方教程
阿里源
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
清华源
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
