如果出现错误faile to fetch镜相源，可以尝试以下操作：

1.备份sources.list：

sudo mv /etc/apt/sources.list /etc/apt/sources.list.bak

2.新建sources.list：

sudo vim /etc/apt/sources.list

复制下面的源到sources.list：

修改成163源：

deb Index of /debian/ stretch main non-free contrib
deb Index of /debian/ stretch-updates main non-free contrib
deb Index of /debian/ stretch-backports main non-free contrib
deb-src Index of /debian/ stretch main non-free contrib
deb-src Index of /debian/ stretch-updates main non-free contrib
deb-src Index of /debian/ stretch-backports main non-free contrib
deb Index of /debian-security/ stretch/updates main non-free contrib
deb-src Index of /debian-security/ stretch/updates main non-free contrib

修改成清华源：

deb Tsinghua Open Source Mirror stretch main non-free contrib
deb Tsinghua Open Source Mirror stretch-updates main non-free contrib
deb Tsinghua Open Source Mirror stretch-backports main non-free contrib
deb-src Tsinghua Open Source Mirror stretch main non-free contrib
deb-src Tsinghua Open Source Mirror stretch-updates main non-free contrib
deb-src Tsinghua Open Source Mirror stretch-backports main non-free contrib
deb Tsinghua Open Source Mirror stretch/updates main non-free contrib
deb-src Tsinghua Open Source Mirror stretch/updates main non-free contrib

修改成科大源:

deb Index of /debian/ stretch main non-free contrib
deb http://mirrors.ustc.edu.cn/debian/ stretch-updates main non-free contrib
deb http://mirrors.ustc.edu.cn/debian/ stretch-backports main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ stretch main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ stretch-updates main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian/ stretch-backports main non-free contrib
deb http://mirrors.ustc.edu.cn/debian-security/ stretch/updates main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian-security/ stretch/updates main non-free contrib

3、更新源

sodu apt-get update -y