How to increase battery health?

You don't. A battery is a consumable part and its health will decrease over time.

If you've never replaced the battery in that phone, it's likely time to do so.

See here for more info: iPhone Battery and Performance - Apple Support

~Lyssa

You can't magically change the battery health. All rechargeable batteries eventually lose the ability to hold a charge. Get a new battery or a new phone.