Earlier today, Apple announced a battery replacement program for 6S devices. The new program is a response to reports of of iPhone 6S and 6S Plus devices affected by unexpected shutdowns. The malfunctioning batteries do not affect the devices’ safety, in contrast to the Samsung Galaxy Note 7.
The shutdown issue only applies to a “small amount” of devices that were manufactured between September and October of 2015. If a 6S user suspects their device may be running on a malfunctioning battery, they can take their device to the nearest Apple store and check the device serial number. If their device number is part of the recalled batch, they qualify for a new battery.