edit /etc/apt/sources.list

deb http://mirrors.ustc.edu.cn/debian stable main contrib non-free
deb-src http://mirrors.ustc.edu.cn/debian stable main contrib non-free
deb http://mirrors.ustc.edu.cn/debian stable-updates main contrib non-free
deb-src http://mirrors.ustc.edu.cn/debian stable-updates main contrib non-free

deb http://mirrors.ustc.edu.cn/debian stable-backports main contrib non-free
deb-src http://mirrors.ustc.edu.cn/debian stable-backports main contrib non-free

deb http://mirrors.ustc.edu.cn/debian-security/ stable/updates main non-free contrib
deb-src http://mirrors.ustc.edu.cn/debian-security/ stable/updates main non-free contrib

Modify or create the file /etc/apt/preferences and put following text in it:

Package: *
Pin: release a=stable
Pin-Priority: 1001

run command

apt-get update
apt-get upgrade
apt-get dist-upgrade