I assume that your iTunes downloaded latest upgrade available for your device. Then bring your device into DFU mode by following the steps below and then wait until iTunes detect your device in recovery mode, then wait for it to complete the recovery process. That's it. Your upgrade will be done.
- Plug your device into your computer.
- Turn off the device.
- Hold the Power button for 3 seconds
- Hold the Home button without releasing the Power button for 10 seconds
- Release the Power Button but keep holding the Home button
- Keep holding the Home button until you are alerted by iTunes saying that it has detected a device in Recovery Mode
Make sure the device screen is blank and no logos are present
Wait for the process to complete. Your job will be done. I am able to upgrade using the above process which initially wasted my whole day with same -1 error code in recovery process.
All the best.