The only way is if you do a full restore. Back up all your stuff & do a full restore, (hold home & lock button, untill your phone turns off, and only release the lock button. Connect it fast to your computer. Keep holding home until you see a logo saying connect to iTunes) now try restoring your iphone.
As soon you have a fresh iOS installed, jailbreak it, then finally restore from your backup.
As soon you have a fresh iOS installed, jailbreak it, then finally restore from your backup.