Battery health degrading.How to restore the health,or change the battery

Battery health cannot be manipulated. It is an approximate percentage and, most of all, it is related to when the battery (not the iPhone) was new.

That said, see this interesting support article about that:--> iPhone Battery and Performance, and/or this one:--> Batteries - Maximizing Performance, and read this useful user tip carefully:--> When to charge your iPhone or iPad - Apple Community.

If your battery's Maximum Capacity in Settings>Battery>Battery Health is close to 85-80%, have it replaced at an Apple Store or Authorized Service Provider.

Get an extimate cost for your unidentified iPhone model (valid in US Apple Stores, tax excluded, work included) by opening this support page:--> iPhone Battery Repair & Replacement

Regards

Giulio

Batteries will degrade over time, there's no way around it, you can't restore its health.

You can maximize its life though, checkout this article it has a lot of tips Batteries - Maximizing Performance - Apple

iPhone Battery and Performance - Apple Support

To Change the battery if it's below 80% , check the nearest Apple authorized service provider or Apple store near you and book an appointment to check service options. Find Locations - Apple