Battery gone bad

How to replace your iPhone 6s Battery

1. Go to https://support.apple.com/iphone/repair/service/battery-power if you live in the US
2. You can see in the pricing table that your repair cost will be $49
3. Click "Start a battery replacement" and walk through the steps of setting up an appointment with an Apple Store or Apple Authorize location near you.

Start a battery replacement (Link)

You make an appointment at the Genius Bar of your local Apple Store to have it replaced or take it to your local authorized service provider.