How do I fix my battery spring corrosion

If you want to clean the corroded spring, you could dip a cotton swab in isopropyl and rub it on the spring, it could help a bit. But I’d recommend buying new battery terminals, depending on which contact is corroded you’ll have to do a little bit of desoldering, as there are 2 contacts in the upper part of the battery case which are soldered to the gameboy’s circuit. The ones circled in the image below to be more precise. The squared ones are just terminals which can be easily popped out of the case and replaced, you can find them easily on the internet.