Apple has initiated a battery replacement program for the iPhone 6S due to issues that affect devices that were manufactured between September and October in 2015.
This is not a safety issue and the affected devices experience unexpected shutdowns.
Apple has identified the serial number range of the iPhone 6S devices which may be affected and set up a page where customers can check if their device qualifies for a battery replacement.
If your iPhone 6S is eligible for battery replacement you can take the device to an Apple Authorised Service Provider, an Apple Retail Store or go through Apple Technical Support.
The iPhone will be examined again before the repair to verify it qualifies for the program and in working order.
Before bringing the iPhone in for repair you need to back up your data to iTunes or iCloud, turn off the Find My Phone function and erase data and settings in Settings > General > Reset > Erase All Content and Settings.
You check your iPhone 6S serial number here to see if it needs a battery replacement.