Downgrading from iOS 8.1 beta 2 to iOS 8.0.2
The only method is this:


Be prepared to LOSE ALL DATA, so do the proper backup.
Turn off the iPhone.
Push the Home button until the iTunes icon on the iPhone appears red.
Connect it to your computer and do the restore.


iTunes will, at this point, download the last official release of iOS. And, keep in mind, you will LOSE ALL DATA.