Jeg brukte nettopp denne metoden på å factory restore en 5s for 3 dager siden. han hadde ikke oppdatert til iOS 8 og dette fungerte gull...

How to Restore or Update your Apple Device

Restoring your device will delete all data and content, including videos, songs, photos, contacts, and other information, and will restore all settings to their factory condition. Updating your device doesn't affect your content, settings or other information. In either case, backing-up your content in iTunes or iCloud is recommended.

After downloading the desired firmware, connect the device to your computer and select it in iTunes. Mac users hold down the Option key, while Windows users should hold down the Shift key. Then click on the Update or Restore button, select the IPSW file you recently downloaded and click Choose. Your device should now begin to update. Take note that certain browsers may change the ipsw-file into a zip-file. If this should occur, just rename it to end in .ipsw and iTunes will recognize it.