iPhone 6s battery low

Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support

For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)

The 6s is now considered vintage so service may not be readily available. Obtaining service for your Apple product after an expired warranty - Apple Support

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support