ive been trying to update my iphone for 2 days now, where this keeps constantly happening to me. it stays at this point for periods over 8 hours long. ive tried redownloading the firmware, restarting my laptop and putting it into dfu mode before hand as well.
Try a different computer. If that still does not work try one of your friends computers. That issue is normally caused by ether a slow Internet connection or a slow computer. But because the iPhone can still communicate to apple it does not give an error message