Sometimes iOS updates go awry. It may happen that the device hangs itself for good, refusing to activate or be recognised by iTunes. In that case, it’s time for a DFU reset, also known as Recovery Mode.
Download an .ipsw file for your device from the Developer Centre (either a beta or the release version) and extract it. Then follow these steps:
- turn off your device
- open iTunes and have a USB cable handy
- while holding the HOME button, insert a USB cable into your device (connected to your computer running iTunes)
- the device should turn on and be recognised by iTunes within a few seconds as a device in DFU mode
- Option-Click RESTORE in iTunes, then navigate to the .ipsw file of your choice (hopefully the correct one for your device)
Smiles all round!