修改 sources.list 文件

sudo 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

清华大学源

deb https://mirrors.tuna.tsinghua.edu/debian/ stretch main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu/debian/ stretch main contrib non-free
deb https://mirrors.tuna.tsinghua.edu/debian/ stretch-updates main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu/debian/ stretch-updates main contrib non-free
deb https://mirrors.tuna.tsinghua.edu/debian/ stretch-backports main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu/debian/ stretch-backports main contrib non-free
deb https://mirrors.tuna.tsinghua.edu/debian-security/ stretch/updates main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu/debian-security/ stretch/updates main contrib non-free

更新

sudo apt update
sudo apt upgrade