Q: How to sync new iphone with backup from old iphone running newer ios?

I backed up my old iPhone 2 days ago. Then it shattered. Received replacement iPhone today. Attempted to set it up and restore from backup. Only to be told that it couldn't restore from most recent backup because it was a more current version of the ios than the new phone had. So I set up using previous backup from over a month ago. (NOT The one I want, but didn't think I should go back and set up as new)

Now that I've gone in and upgraded the ios on the new phone, when I go into backup, it says there are no backups to choose from.

Now I need to know: How do I re-set up this phone to run the most recent ios so it can sync with my old phone that had a backup from 2 days ago?

Thanks!