How to Downgrade Back to iOS 7.1.2 from iOS 8
Be sure to back up your device before beginning this process.
BACK UP YOUR iOS 8 DEVICE before anything else – do not skip this – this insures that you can restore your stuff to iOS 8 if you have a problem with the downgrading process
Download the iOS 7.1.2 IPSW here for your device and save it to an accessible location like the user desktop
Open iTunes on the Mac / PC and connect the iPhone, iPad, or iPod touch with the USB cable
Choose the device in iTunes and go to the Summary tab
Hold down the OPTION key for Mac OS X (use the SHIFT key for Windows) and click on the “Update” button
Navigate to and select the firmware .ipsw file for iOS 7.1.2 that you downloaded
Choose “Update” again, iTunes will verify the update with Apple and the downgrade process will begin – if this failed you either used the wrong IPSW file, or the downgrade signing window for iOS 7.1.2 firmware has closed