How to check if you need a battery replacement: https://youtu.be/nQSmnBN_xKs
This video is a very detailed step by step battery replacement guide for the iPhone 6S and 6S Plus. Due to apple sealing the display to the to casing, replacing the battery in 6S and newer iPhones are much harder than previously. This guide shows you every step of the way, including how to apply the new battery adhesive and screen sealing gasket.
