How to install the final release of iOS 9 on an iPad running iOS 9 Beta
I found this from the Apple support site:

"Download the final version of iOS

If you have an iOS beta version and want to download the final version, follow these steps:

Connect your device to iTunes.

While your device is connected, force it to restart. Press and hold the Sleep/Wake and Home buttons at the same time. Don't release when you see the Apple logo. Keep holding until you see the Recovery Mode screen.

When asked, choose Update to download and install the latest nonbeta version of iOS.

An update install won't affect your content or settings."

I would recommend backing your iOS device up to iTunes before attempting this.