Hi
Apple cables (including the Lightning to USB cable included with iPhone 6s) are covered against defects in materials and workmanship under Apple's Limited Warranty.
If your cable has failed under normal use within the warranty period:
- Contact Apple Support (mail-in service may be available), make a Genius Bar reservation or visit an Apple Authorised Service Provider with a view to having it checked and, if agreed, replaced under warranty.
- Note that the warranty does not cover accidental damage or damage caused by misuse or using the cable outside Apple's guidelines.
If your warranty period has expired:
To check your warranty status (your iPhone warranty covers the included cable):
Apple's handling instructions for the Lightning to USB Cable include:
"Certain usage patterns can contribute to the fraying or breaking of cables. The Lightning to USB Cable, like any other metal wire or cable, is subject to becoming weak or brittle if repeatedly bent in the same spot. Aim for gentle curves instead of angles in the cable. Regularly inspect the cable and connector for any kinks, breaks, bends, or other damage. Should you find any such damage, discontinue use of the Lightning to USB Cable."
http://help.apple.com/iphone/10/#/iphbbe12ba1
"When you unplug a cable from your product, a power source, or an AC adapter, hold on to the connector at the end of the cable. Pulling on the cable itself can strain the connector joints or cause breakage."
Using Apple power adapters, cables, and duckheads with Apple products - Apple Support
Note that you are not talking to Apple here - this is a user-based technical support community.