The patch is a new inline build, with a limited set of changes to address some bugs and a binary package incompatibilities. You can read more inn the official announcement at:
https://blogs.embarcadero.com/rad-studio-12-2-athens-inline-patch-1-available/
If you have 12.2 installed, you should see a GetIt patch notification and follow it to download the new installer. If you don't have 12.2 installed and have a license for it, you can download the new installer at https://my.embarcadero.com/. If you don't have a license, you can download the current trial, updated with the patch.
How to you know if you have the new inline version? Check the version number in the about box, the 12.2 Patch 1 Inline is
RAD Studio 12 Version 29.0.53982.0329