Apple has identified an issue with a small number of iPhone 6s handsets causing them to unexpectedly shut down. There are no safety concerns. However, Apple will be offering battery replacements to affected handsets free of charge. For more information please read the full statement on Apple's website.
If you've experienced this issue, you'll need to visit an Apple Retail Store or an Apple Authorised Service Provider. They'll check your device to confirm if you're eligible for a free battery replacement.