If you have an iPhone 6s that shuts down unexpectedly well before you reach 0%, Apple might repair your iPhone for free even if you are past warranty. Note that this is specifically for the 6s only. Not the 6, not the 6s Plus. Check your iPhone’s serial number against the database. Then back up your iPhone and book an appointment at the Apple Store. If you aren’t near an Apple store you can also contact Apple Support and get the repair done by mail. Third party retailers will not service it under warranty, so AT&T, Verizon, Best Buy, etc. won’t be of much help. You need to go directly through Apple or through an authorized service provider (a company that is authorized by Apple to repair their hardware).
For details, and to see if your serial number is in the list of affected models, visit Apple’s article on the problem.
Just because your serial number comes up a match on their tool does not mean you are necessarily entitled to a repair. Only if your iPhone is prematurely shutting down.
Apple has extended the original warranty to three years from the original sale, but only for this specific issue.
This has been an admitted issue for a couple of weeks, but just yesterday Apple added a tool that allows users to see for themselves if their iPhone 6s serial number is on the list.
UPDATE: A friend pointed out that you should check to see if the store has batteries in stock. Because of the sudden rush stores might be out of inventory.
@jimmymacsupport but check with the store you're going to bring it, to be sure they have the battery. Yesterday Bethesda Apple store, not!
— Jonathan Bernstein (@dccampfin) December 2, 2016