Steve, Plenty of tutorials on line. Go to you tube and search make and model and changing magazine catch. You will see how, If it is a Glock. Remove magazine and you will see a magazine catch spring on inside of grip. Remove that with screw driver and needle nose pliers and remove mag catch button, reverse it and reinstall spring. Good luck.

Steve, depending on the firearm, there is a very high chance of finding a you tube video on the process. Cheers and stay safe!

if you list what gun we would be able to give you better advice