Please refer to the ZorinGroup guide above for the procedure to perform a Direct Upgrade for a Zorin OS release.
Remember, this is for upgrading the Major Release of Zorin OS without needing to reinstall. You can upgrade from Zorin OS 15 to Zorin OS 16. Or Upgrade from Zorin OS 16 to Zorin OS 17 (When it is released.)
Core and Lite users can also use the Upgrader to upgrade from Core or Lite to Pro or Pro Lite.
This tool is not for running standard package upgrades or for Point Releases (for example, Zorin OS 16.1, Zorin OS 16.2...) These are Point Releases and upgrades happen with your already in place