I had this same problem. I never downloaded or used iTunes on my pc, but downloaded it and made sure it was current. Make sure iTunes is open on your pc/mac and plug in your iphone 6s. Reset your phone by holding the home button and sleep/wake button on top right of phone at the same time until you see the apple icon appear. Once you get to the point that is says "upgrade complete, but then said "there's a few more steps to follow, then you're done" DONT press continue. Wait for iTunes to detect that your iphone 6s has an outdated version of iTunes. It will ask you if you want to upgrade and you answer yes. It will the allow you to follow through with remaining setup of your phone. It only took me 3 1/2 hours to figure this out, but it works now.