I'd think that the update procedure would take a back up BEFORE doing the iOS update... If the update caused a problem with your data then you'd have a stuffed backup surely.
However doing a backup doesn't back up iOS, the backup is only of your data.
To restore iOS back to another version you should hold down ALT and click Check for Update and select the file that is the older version of the OS. Then you can restore your data to the phone.