Q: I want to prompt a backup from a newer iphone to an older phone and it is saying that the iOS is not compatible. How can I update my old device so that I can restore with the latest backup?

I am trying to restore information on an older Iphone and am running into the difficulty of restoring my backup. The backup says its incompatible with the iOS on the old phone so it will not let me choose the latest iphone backup. Is there a way to update my old iPhone before I restore that backup? I already have tried to sync it with the latest version of ITunes. I am nervous to restore to an older backup in case the newer backup will not show up again.