Do I have to / is it possible to update to iOS 8.0 if I preinstalled device with a 8.0 beta? I am a developer, and my beta iOS is really unstable, is it possible to update it to the real iOS8 that coming out today?

Do you know is iOS8 available for download in Settings yet?
Yes, It will be possible to update to the iOS 8 public when Apple releases to the public. The update could be done via iTunes or OTA.