Source list
vim /etc/apt/sources.list
deb http://ftp.fi.debian.org/debian/ stretch main non-free contrib
deb-src http://ftp.fi.debian.org/debian/ stretch main non-free contrib
deb http://security.debian.org/debian-security stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security stretch/updates main contrib non-free
# stretch-updates, previously known as 'volatile'
deb http://ftp.fi.debian.org/debian/ stretch-updates main contrib non-free
deb-src http://ftp.fi.debian.org/debian/ stretch-updates main contrib non-free
# stretch-backports, previously on backports.debian.org
deb http://ftp.fi.debian.org/debian/ stretch-backports main contrib non-free
deb-src http://ftp.fi.debian.org/debian/ stretch-backports main contrib non-free
Install Intel 9260 wireless
apt-get update
apt-get -t stretch-backports install firmware-iwlwifi
modprobe -r iwlwifi ; modprobe iwlwifi