Is it possible to fix iPhone battery?

iPhone 6s Plus, iOS 15

If you are still on iPhone 6S Plus, iPhone 6S Plus is > 8 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.

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Considering a newer type of iPhone?

Read this before you buy a newer type iPhone.