Rearranging the apps to their previous layout can be achieved by restoring from a prior backup. This process reinstates not only the apps themselves but also their original positions on the Home Screen, assuming the backup was made when the desired layout was in place.
Even if rolling back the iOS version were an option—which, to be clear, it is not officially supported by Apple—you still would not be able to recover your exact app arrangement unless you had a backup from that earlier state. The iOS version alone does not store your Home Screen layout; that information is preserved within the backup itself.