Unable to Install Update; An error occurred installing iOS 8.2
Power off the phone, and hold down the power button and home button at the same time until the device powers on.  At that time, release the power button, but continue to hold the home button, which should start your phone in recovery mode and show an iTunes/USB Plug icon on the screen, which indicates for you to plug it into your computer with the USB data cable (computer with iTunes installed on it and open.)

Once you plug in the USB data cable to your phone/computer with iTunes, it should detect a phone (your phone) in recovery mode, and you should be able to fully recover your phone by restoring the latest iOS version to it.