DFU (Device Firmware Upgrade) mode is not the same as restore mode. DFU mode bypasses the current OS installed and allows you to upgrade or downgrade your OS. If you are getting errors trying to restore in iTunes, DFU mode is very likely for you.
This is how to achieve DFU mode. You can do it on every phone:
* Attach the phone to the PC
* Turn the phone off
* Hold power and home together for *exactly* 10 seconds
* Release power but keep holding home until the PC beeps as a USB device is recognized.
* At no point will the display come on. Now your restore should work.
NOTE : It may take a few attempts to get your iPhone into DFU mode. Generally, I hold down both buttons then release the Home button just before I think the Apple logo would appear. If you are still holding both buttons down and you see the Apple logo you are holding them down for too long!
Step 1. Connect iphone to USB then listen to the sound when it connect and disconnect.
Step 2. Hold the home and power buttons until hear the disconnect sound then release power button.
Step 3. Wait until you hear another sound then release home button.
Alternate Method: Going Into DFU Mode Without Button Pushing: (on windows) Easier than timing the buttons just right!
1.) First download the required archive...
2.) Extract the archive to the root of your hard drive, should be C:/DFU/
3.) Connect your iPhone and ignore iTunes
4.) Goto Start - run
5.) Next type in “c:/dfu/dfu iBSS.m68ap.RELEASE.dfu”
6.) Wait for iPhone to show a “white” screen
7.) Now you may proceed to downgrade!