If you're a public beta tester (who hasn't signed up to be full-fledged developer), you can downgrade your iDevice by putting it into DFU mode. (DFU stands for Device Firmware Update.) You use this method to restore iOS 8 without having to get the older operating system manually.
First, perform a backup via iCloud or iTunes. Even though you won't be able to use this data on iOS 8, it's always better to have a backup than not. Then go to Settings: iCloud: Find My iPhone and turn off Find My iPhone.
Then follow these instructions to put the iPhone into DFU mode: Turn off the iPhone and plug it into your computer. Hold the Home button down while powering on the phone, and hold both until you see the Apple logo disappear. You can release the power button, but continuing holding down the Home button until you see the iPhone's screen display instructions to plug the device into an iTunes-compatible computer. When prompted on your computer, click on the option to Restore, and iTunes will download the latest released version of iOS for your device.
If you're a developer, log into the Apple Developer portal (after you turn off Find My iPhone), click on the section for iOS and download the latest officially released build. As of now, that's iOS 8.4. Once the software is downloaded, open iTunes and click on the iPhone/iPad/iDevice tab. Within the Info tab, there are two buttons: Update and Restore. Hold down the Option button on the keyboard while clicking Restore. Navigate to the file that was just downloaded and select it. The software will then erase the iPhone or iPad of its contents and install that previous version of iOS.
Note: When downgrading to the previous version, make sure to option-click Restore; do not choose Update. Doing that will lead to a loop in which the iPhone is placed in Recovery mode, iTunes attempts to download and install the latest official build, runs into errors, and then attempts to download another copy of the official build. It will do that until you break the cycle and choose to Restore the device. So again, don't select Update.
Given that Apple software upgrades now routinely roll out in the fall, upgrading your devices to unstable software isn't a good way to spend the summer. For most people, I'd recommend waiting. The latest features are really only worth having when your device is stable, especially if it's something you rely on day in and day out. But if running the latest software is your thing, then by all means, have at it. And at least if you run into problems on your iDevice, you now know how to get out of trouble.
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