1) download directly the official firmware
http://www.iclarified.com/750/where-to-download-iphone-firmware-files-from
Remember where you saved this.
2) Put phone after you finished downloading into DFU mode, open iTunes, and i will ask you to restore.
3) On PC Hold SHIFT+ Click the restore button. On MAC, Hold OPTION + Click the Restore button.
4) Itunes will now let you choose an IPSW file. Choose the one you downloaded in Step1. Restore should then start.