编辑文件
vim /etc/apt/sources.list添加镜像源
deb http://mirrors.ustc.edu.cn/debian stretch main contrib non-free
deb http://mirrors.ustc.edu.cn/debian stretch-updates main contrib non-free
deb http://mirrors.ustc.edu.cn/debian stretch-backports main contrib non-free
deb http://mirrors.ustc.edu.cn/debian-security/ stretch/updates main contrib non-free更新apt
apt-get update