How do I put my iPhone 3GS into DFU / recovery mode?
To put your iPhone into recovery (DFU) mode:
1. Make sure the iPhone is off and unplugged from your computer.
2. Hold down the home button.
3. While holding down the home button plug the iPhone into your computer.
4. Continue to hold the home button until the Apple logo disappears and the iTunes logo appears.
5. Now you can let go of the home button.
6. iTunes will say it has detected an iPhone in recovery mode.