How to downgrade your iPhone/iPad from iOS 9 beta to iOS 8.3
So, you've jumped on the iOS 9 beta train? Despite that is nowhere near suitable for a daily driver, the developer preview is doing its job impeccably, i.e. it's giving us a sneak peek at most of the features that will appear with iOS 9 later this year. Still, as we said, it's full of bugs and is not battery-optimized at all.
2. Connect your iDevice to your PC/Mac and open iTunes.
3. Now, we need to get into DFU (Device Firmware Update) mode. To do so, hold your iDevices' power and home buttons simultaneously for 10 seconds. Then, release the power button, but continue pressing the home button.
4. A notification message like the one right below should pop up in iTunes. It informs you that you've successfully put your iDevice in DFU mode and need to restore it before you can use it. Tap "OK".
5. Now, simply choose the "Restore iPhone" option (or iPad, for that matter). iTunes will then ask you to confirm your choice to restore the device to an earlier firmware. After that, it's just a matter of tapping "Next" and "Agree", followed by a brief wait for iTunes to do its magic.