Have you tried using the restore option? connect your computer to the iphone 6 using a cable and boot the iphone 6 into recovery mode, then on windows download itunes and open it then click "Restore" then wait for it to complete Or if you have a older mac use itunes the same way or newer macbook use finder and click "Restore" this should fix the software after it is done[br]
Let me know if there's any developments! Cheers!