Battry health

Batteries degrade over time with usage, that's just how they work. You can expect about a 1% drop per month with average use on an iPhone.

Apple recommends replacing it when it reaches 80%

Here are some tips on maximizing battery life.

https://www.apple.com/batteries/maximizing-performance/

You are addressing this to another user and not to Apple in this forum.

Apple Support Community Forum

This is Apple Support Community forum, primarily for a user to user interaction for information & idea sharing between users, if need be, extend support on technical know-how in handling Apple products from a more experienced fellow user, This is not the right forum to discuss with Apple Inc.

No, you cannot reset it to 100%, that's not how it works. Like said in my first post, batteries degrade with use. It will never be 100% again.

No, it will continue to degrade with use. When it reaches 80%, take it to an Apple store and have it replaced.

You're welcome.