Ubantu系统更换国内源
1.备份原始源文件sources.list,桌面打开终端，执行命令：
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
输入密码后
2.修改源文件sources.list
sudo gedit /etc/apt/sources.list
3.将原有的数据全部删掉，换上国内镜像源,然后保存（我这里只换了阿里源）
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
浙大源：
deb http://mirrors.zju.edu.cn/ubuntu xenial main universe restricted multiverse
deb http://mirrors.zju.edu.cn/ubuntu xenial-security main universe restricted multiverse
deb http://mirrors.zju.edu.cn/ubuntu xenial-updates main universe restricted multiverse
deb http://mirrors.zju.edu.cn/ubuntu xenial-backports main universe restricted multiverse
deb-src http://mirrors.zju.edu.cn/ubuntu xenial main universe restricted multiverse
deb-src http://mirrors.zju.edu.cn/ubuntu xenial-security main universe restricted multiverse
deb-src http://mirrors.zju.edu.cn/ubuntu xenial-updates main universe restricted multiverse
deb-src http://mirrors.zju.edu.cn/ubuntu xenial-backports main universe restricted multiverse
4、然后执行以下命令切换为root账户
sudo -i
5、执行更新命令
apt update
apt upgrade
最后分享一个浙大的开源镜像站
浙江大学开源镜像站：http://mirrors.zju.edu.cn/