Unable to restore from iTunes back up
I have backed up my old iPhone 4 to iTunes (on a Windows 8.1 PC, iTunes V.12.1.2) and now want to restore that back up to a new iPhone 6S. But, when I plug in the iPhone 6S to the PC using the supplied cable I get a message saying that the iPhone cannot be used because it requires a newer version of iTunes. When I go to the iTunes website on the iPhone (via Safari) and try to download, I then get a message saying iTunes already installed. I figure it is at latest version as only collected it yesterday. Also, PC tells me that 12.1.2 is the latest version for that.
So, any ideas what to do next? I'm lost without my contacts (for work purposes) and cannot go back to the iPhone 4 as the sim has been disabled (due to swapping from mini to nano sim today).
iPhone 6s Plus, iOS 9.0.2