Source : https://www.linode.com/docs/security/upgrading/upgrade-debian-to-the-newest-release/
Start a screen session is recommended to ensure that the updates will continue to install in the unlikely event you are disconnected during the upgrade process.
Install screen package :
apt-get install screen
After installation has completed, start a screen session :
screen
If you are disconnected from your server, you can reconnect to the screen session by entering the following command :
screen -Dr
apt-get update
apt-get -y upgrade
dpkg --audit
dpkg --get-selections | grep hold
Before upgrading your system, it is strongly recommended that you make a full backup, or at least back up any data or configuration information you can't afford to lose.
It is recommanded to stop some services like Apache, MySQL, Nginx, etc... Some services can also be disabled & re-enabled after upgrade.
Edit /etc/apt/sources.list file (adapt to your needs) :
deb http://ftp.fr.debian.org/debian/ stretch main non-free contrib
deb-src http://ftp.fr.debian.org/debian/ stretch main non-free contrib
deb http://security.debian.org/ stretch/updates main non-free contrib
deb-src http://security.debian.org/ stretch/updates main non-free contrib
# stretch-updates, previously known as 'volatile'
deb http://ftp.fr.debian.org/debian/ stretch-updates main non-free contrib
deb-src http://ftp.fr.debian.org/debian/ stretch-updates main non-free contrib
apt-get update
apt-get install apt dpkg aptitude
apt-get dist-upgrade
apt-get autoremove
dpkg --audit
dpkg --get-selections | grep hold
Some packages may require to be MANUALLY reinstalled or removed if deprecated/conflicted.
systemctl set-default multi-user.target
Note: to restore GUI boot
systemctl set-default graphical.target
reboot
After reboot, services previously disabled can be re-enabled (if any).