If it is Aperture 3, you can upgrade to the newest Aperture 3 version simply by selecting Software update from the Apple menu - you do not have to buy it, if you have Aperture on your machine. For software update to work, you should move the Aperture application to the Applications folder, if you keep it in another folder, and it needs to be named simply "Aperture".
As a precaution, before you upgrade from a much older version than the current version:
- Write down the serial number of Aperture (if it is a pre-App Store version): You will find it in the "About" panel in Aperture's main menu bar: Aperture > About Aperture.
- Create a current backup of your system - it should include your current Aperture version and the Aperture library.
- Then run the Aperture Library First Aid Tools, to repair the Aperture Library,
see Repairing and Rebuilding Your Aperture Library: Aperture 3 User Manual
The repair before upgrading is important, because Aperture will upgrade the library to a new format, and it is safer to ensure that the library has no corruption.
It's definitely Aperture 3 and I think I've updated it as much as I can.
My main concern is that it's not MINE, meaning I didn't buy it, it was just on my machine. So I'm not quite sure what kind of version it is and I want to make sure I can update it going forward. On top of that, I was going to purchase the newest MAS version just in case anything ever happens, for example, if I need to reinstall it, wipe my machine, or just upgrade it in the future.
So I'm just trying to figure out if I can just delete the app and then download it from the MAS or if I need to export the files first or do something else.
Hope that makes sense.
With the serial number you will be able to reinstall and update Aperture. Use the Trial Installer, as described by Apple here: Aperture 3.4: May quit unexpectedly on launch after updating
But If you think the owner did not transfer the rights to Aperture to you together with the Mac, then buy it from the App Store.
Follow the plan I outlined above: Make a current backup, and then repair the Aperture library.
Move the Aperture application to the trash, but keep the library in Pictures folder. Then launch the AppStore, and buy Aperture.
Afterthe installer finishes, double click your Aperture library to launch Aperture on this library.