@kina, you don’t have to pay anything if all you want to do is change the resolution of your photo… Just edit your photo to your liking with whatever software you currently use, then export it. Then use some free software to change the resolution to whatever you want it to be. Normally I would recommend FastStone Image Viewer but since you’re evidently using an Apple product, I’ll have to recommend an alternative. 🙂
Check out XnViewMP for a free (for personal use) basic editor that will easily allow you to resize (a.k.a. change the resolution) of your pictures. I just downloaded it for Windows and gave it a test run, and it seems to have much of the functionality that FastStone does. You can even do batch conversions if you need to resize multiple files at once.
Of course you’re welcome to pay for Affinity/Luminar if you want to, but hopefully you’ll be paying for more than just a photo resizer because there are lots of free programs that can easily do the job.