I think I am done with my problem I copied the factory backup files from the dell backup and recovery app to 16GB USB and the i tried to reset the windows firstly it was asking to put a recovery media in laptop to reset windows then i inserted that USB and here i am done with my problem...but make sure that you have to make permanent usb for that process the actual size was 12.74gb of factory backup that's why i used 16gb usb...and problem solved....now i can again update to 8.1......