修改 sources.list 文件
nano /etc/apt/sources.list
把官方源前面加上#号注释掉，最下面加上下面的清华大学源列表
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch main contrib non-free deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-updates main contrib non-free deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-updates main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-backports main contrib non-free deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ stretch-backports main contrib non-free deb https://mirrors.tuna.tsinghua.edu.cn/debian-security/ stretch/updates main contrib non-free deb-src https://mirrors.tuna.tsinghua.edu.cn/debian-security/ stretch/updates main contrib non-free
