You might be eligible for a free battery replacement for your iPhone 6s.
If your iPhone 6s has been shutting down unexpectedly, Apple has a new service program aimed at fixing the problem. The company says that it only affects a limited range of iPhones manufactured between September and October 2015.
If you have experienced this issue, please visit an Apple Retail Store or an Apple Authorized Service Provider and have your device's serial number checked to confirm eligibility for a battery replacement, free of charge.
Note that you must go to an Apple Retail store or authorized service provider, as wireless carriers are not participating in this program. Before you take your iPhone in for this battery replacement service, you'll want to back up your iPhone to iTunes or iCloud, as you'll need to erase the data and settings on your phone and turn it off.