Phone replaced under warranty but can't restore bc backup not compatible with iOS on new phone (6.1.2 old phone vs 6.0.2 on new) iPhone 5

I got my phone replaced under warranty last night and didn't have time for the genius bar to restore it for me. Now I can't restore from the backup of my old phone because it was running iOS 6.1.2 and this new phone is on 6.0.2. The setup process has the most recent backup grayed out saying its not compatiblemw the new phone. How do I update the iOS on theis new phone so I can then restore the backup from my old?