Upgrade from Debian 7 to Debian 8 to Debian 9

Chapter 4. Upgrades from Debian 7 (wheezy) - debian.org
How to upgrade to Debian 8 Jessie from Debian 7 Wheezy
Upgrade Debian 7 (Wheezy) to Debian 8 (Jessie)
Upgrade Debian 8 (Jessie) to Debian 9 (Stretch)