When purchasing the latest version of Zorin OS (e.g. 18 currently) it includes complimentary upgrades to previous versions so that a user can for example upgrade from 16 to 17 Pro and then from 17 Pro to their purchased 18 Pro.
In your case, as you already have a copy of Zorin OS 18 Pro you can simply enter in it's Support Code to carry out all the upgrades.