Need new battery for i phone 6

iPhone Battery & Power - Official Apple Support

Yes, Apple replaces iPhone batteries.

I recommend to Contact Apple for support and service - Apple Support to set this up. Remember to backup your iPhone prior to sending it in or booking your Apple Retail Store appointment for the Genius bar. They can also verify that replacing the battery will even resolve your problem prior to replacing it. That is of no charge, so if you are outside of your warranty, it will be nice to know you aren't just wasting money on something that could have been resolved through software.

Frejen wrote:

Can i buy a battery from Apple.

You cannot simply buy the battery. You can, though, have Apple replace the battery for you as noted above.

It's highly unlikely that you need a new battery; the 6 isn't old enough to have battery issues. But you can check by taking the phone to an Apple store, where they can test the battery. Or you can get the Battery Life app from the App Store and check the battery yourself.