Check if you are using obsolete packages aptitude search ‘~o’If this gives you output you will have to take into account that these packages are no longer in a standard repo since they were removed.
Perform a full backup – does that realy need stating ?
warningUpgrading debian from 8 to 9 will replace mysql with mariadb, which will also automatically upgrade your databases. If you get stuck somewhere in the upgrade process you cannot revert the database conversion.
Get your current Jessie system fully updated first:apt-get update
apt-get upgrade
apt-get dist-upgrade
Check packages that are put on hold for whatever reason, thay may bite you later. apt-mark showholdand perform a consistency scandpkg -C
Update the package source from the old to new version (jessie to stretch) sed -i ‘s/jessie/stretch/g’ /etc/apt/sources.list
Update the package list, and verify which packages will be installed as part of the upgrade
apt-get update
apt list –upgradable
Perform the actual upgradeapt-get upgrade
apt-get dist-upgrade