If your iPhone 6S has been shutting down unexpectedly, Apple may have you covered. The company has opened up a new repair program for last year's iPhone in which eligible customers can have their device's battery replaced at no extra cost.
According to Apple, the iPhone 6S' battery problem is "not a safety issue" (because that's important to point out these days) and only affects a small amount of handsets that were manufactured between September and October 2015.
To get your iPhone 6S' battery replaced, you can visit an Apple Store or Apple Authorized Service Provider, or get in touch with Apple Technical Support online. Regardless of which option you pick, your phone will be examined before any repairs to make sure it's both eligible for the program and working properly (outside of the battery issue, of course).
Before your bring in your iPhone 6S for repairs, Apple suggests that you back up your data to iTunes or iCloud, turn off find my iPhone and erase all data on your phone.
Apple's replacement program covers affected iPhone 6S models for "3 years after the first retail sale of the unit." If you've experienced this issue on your iPhone 6S and paid for a battery replacement, Apple encourages you to get in touch about a refund.