Apple Offers Battery Replacements for iPhone 6s Units That Sporadically Shut Down
Back up your data, then head to your nearest iPhone retailer for a fix.
According to Apple, a "very small number" of iPhone 6s phones shut down for no apparent reason. Fortunately, the company has a fix in place (via The Verge).
iPhone 6s devices manufactured between September and October of 2015 are the models most commonly afflicted by this issue, and are eligible for a free battery replacement.
If your phone keeps shutting down, back up your data and disable Find my iPhone, then head to your nearest Apple Store or Apple-authored service provider and ask them to check your device's serial number.
Devices suffering from unexpected shutdowns and manufactured outside the September-October 2015 window are not eligible for free battery replacements. Additionally, unrelated problems such as cracked screens will not be fixed as part of the service.
If replacing your battery doesn't cure your sporadic-shutdown woes, you'll be eligible for a refund on your device.