The Zorin Upgrader App is for Upgrading from one major release to the next:
e.g. Upgrade from Zorin OS 15 to Zorin OS 16
Or to Upgrade a current build to Pro:
e.g Upgrade Zorin OS 16 to Zorin OS 16 Pro
To upgrade your system on a Point Release, your standard upgrader path is all you need. From App Menu, launch the app named Software Updater or in terminal, run
sudo apt update && sudo apt dist-upgrade
Either of these two actions will upgrade your system to the point release of 16.3 from 16.2