How do I get my battery replaced

You can get in touch with Apple Support and arrange a repair, you can also download the Apple Support app and chat with the support team. Apple considers batteries lower than 80% to be replaced with Apple Care+ but probably that’s not your case

Paultodaro wrote:

My battery only charged to 82%

It's not clear if you mean that the battery will not charge beyond 82% or if you mean the maximum battery capacity is 82%. Can you clarify? To determine the maximum capacity, go to Settings>Battery>Battery Health. If that is what is at 82%, I would recommend contacting Apple. They can run diagnostics that are more accurate than the reading on the phone. If they determine it needs to be replaced, you can make arrangements to do so.

Start here:

iPhone Battery Repair & Replacement - Apple Support

On the other hand, if you mean that, when you plug the phone it, it won't charge past 82%, you may have Optimized battery charging enabled and it may not quite have figured out your routine.

See this article for detailed information on optimized battery charging:

When to charge your iPhone or iPad - Apple Community