Before you read this and jump the gun, Apple has specifically mentioned on their site that this is not a safety issue. The company has determined that a very small number of iPhone 6s devices may unexpectedly shut down because of a fault with the battery. However, the problem isn’t widespread and only affects devices within a limited serial number range that were manufactured between September and October 2015.
If you happen to be one of those who has experienced this issue, please use the serial number checker on Apple’s support site to see if your iPhone 6s is eligible for a battery replacement, free of charge. If your phone belongs to the faulty batch, you’ll then have to visit the nearest Apple authorized service center, to have the battery replaced.
However, your iPhone will be examined prior to any service to verify that it is eligible for this program and in working order. And if your iPhone 6s has any damage, such as a cracked screen which impairs the replacement of the battery, that issue will need to be resolved prior to the battery replacement. In such cases, there may be a cost associated with the repair. If everything’s well, you will have to back up your data to iTunes or iCloud, turn off the find my iPhone feature and erase data and settings in Settings > General > Reset > Erase all Content and Settings.
If you happen to be among those who are reading this now and have paid to replace the battery for this particular issue, you can contact Apple through a link on the same support page for a refund.