When I installed iOS 10 on my iPhone 6, the battery life was awful. I noticed that Siri was using most of my battery while the phone was sitting idle over night, and I never use Siri. You might have some other process or app draining your battery. After a couple of days, the battery life improved but then got bad again. Siri was the culprit again. After a week, I reverted back to iOS 9.3.5 and all was well. Not everyone has had a bad experience with iOS 10 on the iPhone 6, but it could be dependent on which apps and settings are on a particular phone. I eventually traded in the 6 for a 7, and the battery life on the 7 has been very good with iOS 10.