I was researching this issue online since I am upgrading from an iPhone 6 which was running iOS 9.1 to an iPhone 6s and wanted a clean install from backup. This forum post was the top trend on Twitter for iOS 9 and I absorbed much of the info. I'd like to share my experience as I believe it will help everyone trying to go from 9.1 beta to 9.0 general release.
First, I plugged my iPhone (in recovery mode) into my computer running iTunes 12.3. It recognized the phone and prompted me to update or restore. I chose update. It gave me a scary message warning that a restore may be necessary if the update fails. I proceeded anyway, knowing I could go back to the beta with my backup if it failed. It went through the process, took about ten minutes, and updated! I am now on the 9.0 general release (13A344). I did absolutely nothing to prepare as far as downloading a software profile from anywhere or even deleting my beta profile beforehand. The only issue I've encountered has been a software update showing before I deleted the profile. Now I have a badge app icon over the settings app showing an update is available, but actually entering software update shows no updates available.
In short: The way to go backwards from iOS 9.1 to 9.0 is simple. Delete the beta software profile first. Then connect to iTunes in recovery mode. Then select update. That's it. It does work and you lose no data. I'm quite pleased to be ready for a clean install on my 6s!