iPhone 6 plus won't update past iOS 8.4.1

I'm currently using my dads old iPhone 6 plus he got a new one because his phone wouldn't update past iOS 8.4.1. I am trying to update the software to iOS 9.3.1 and it is able to download but as I try to install it it says there's an error installing the software and then stops. I have 7GB left on the phone and the update is only 1 or 2GB. I hope someone can help me out unless it is unfixable. I doubt there is a way to get around it or else Verizon would have he