If you are already jailbroken, there is a good chance you’ll need to unjailbreak first, update to iOS 8.2 Beta 2, then rejailbreak from there. If you are not jailbroken already, just skip this section.
Download your current iOS firmware from this site here (make sure it’s the one that matches your device, use this chart for help).
Turn off the iPhone.
Hold down Power and the Home button at the same time until you see the screen go from the Apple logo to black (approx. 10 seconds) then let go of the Power button but continue to hold the Home button until iTunes pops up a dialog box saying it has discovered an iPhone in recovery mode.
Click Restore in iTunes and let it restore your phone (this will also update you to iOS 8.1.3). (If you are reading this after iOS 8.2 comes up, then do not, do this part as it will update you to 8.2 and you could potentially lost the ability to jailbreak again –just confirm when restoring that it’s install 8.1.3 and nothing else).