My battery health is very bad please help me please.

iPhone 6s, iOS 15

iPhone 6s is > 7 years old, which also affects the battery life. The hardware remains old, so does the battery.

If it is still possible, have the battery replaced.

The battery can be replaced by Apple or an Apple Authorized Service Provider.

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE

note:

You have to decide for yourself whether it is still worth having the battery replaced.

Here also an iPhone 6S, I had the battery replaced in October 2021.

I now have to charge the battery every 2 days.