Using iOS 7 as precident, today will be the last day in which you can downgrade to iOS 7.
How to downgrade:
http://ios.wonderhowto.com/how-to/downgrade-iphone-ios-7-1-2-from-ios-8-0157487/
How to downgrade:
http://ios.wonderhowto.com/how-to/downgrade-iphone-ios-7-1-2-from-ios-8-0157487/
How to Downgrade Back to iOS 7.1.2 from iOS 8
Be sure to back up your device before beginning this process.
Source
- 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