Apple has stated on its website that iPhone 6S units manufactured between September 2015 and October 2015 might be eligible for a free battery replacement.
Unexpected shutdowns may occur on iPhone 6S units manufactured within this period. This appears to affect a small number of devices with specific serial numbers, and Apple has stated that it is not a safety issue. (As in, no exploding batteries, guys!) However, its worth taking your phone over to an Apple Authorised Service Provider just in case your phone is covered.
The serial number will be checked, to see if your phone is part of the batch eligible for a free battery replacement. iPhone 6S units that are repaired will not have their warranty extended but the battery will be covered under a 3 year warranty from initial date of purchase.
Apple has also stated that if the phone is damaged in such a way that it’s not possible to replace the battery without resolving the issue first, that issue will have to be resolved before the battery can be replaced. In certain cases, you may need to pay for that particular repair. (For example, a cracked screen)
Apple has stated the following guidelines before you head down to the Service Provider:
To prepare your iPhone 6s for the battery replacement process, please follow the steps below: