My apple mini iPad has an update of 12.1 but I need 15.2 due to some app that I needed to download.

What exact model of iPad Mini do you have? click here ➜ Identify your iPad model - Apple Support

You need an iPad Mini 4th Generation from 2015 at least, to run iOS 15 or later. If you have an earlier model, then its too old to run iOS 15. If you require iOS 15, you'll need to get a newer iPad model.

That model cannot be updated beyond iOS 12. It lacks the hardware, RAM and CPU to run anything newer.

Sorry.

12.5.7 is the most current iOS version it can run.