Why does my battery on my iPhone 6 run down by itself?

I updated my iPhone to the iOS 10 update and ever since then the battery hasn't been good and it runs down when I'm not even using my phone. I don't understand why because I turned things off in settings to try to help the battery. I turned off all notifications and I turned on auto brightness and I also turned off the spotlight search bur that didn't help the battery at all. I've also tried charging my phone and then turning my phone off and turning on airplane mode and then I would turn my phone off again and turn off airplane mode but that didn't help either. I've had this iPhone for a little over 2 years though.

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