Use no aruqivo ´/etc/apt/source.list' as seguintes linhas.

deb http://ftp.br.debian.org/debian/ stretch main contrib non-free
# deb-src http://ftp.br.debian.org/debian/ stretch main contrib non-free

deb http://ftp.br.debian.org/debian/ stretch-updates main contrib non-free
# deb-src http://ftp.br.debian.org/debian/ stretch-updates main contrib non-free

deb http://security.debian.org/ stretch/updates main contrib non-free

deb http://security.debian.org/ stretch/updates main contrib non-free 


deb http://deb.debian.org/debian/ stretch main contrib non-free
deb http://deb.debian.org/security/ stretch/updates main contrib non-free

Após salvar rode os seguintes comandos:

apt-get update
apt-get upgrade

Fonte: https://wiki.debian.org/SourcesList