To put your iPhone 4 or 3GS into Device Firmware Update (DFU) mode:
1. Connect your iPhone to the computer and start iTunes.
2. Turn off your iPhone by holding down the Sleep button and slide to power off.
3. Press and hold down the Sleep button and Home button simultaneously.
4. After 10 seconds release the Sleep button but continue to hold the Home button down.
5. iTunes will pop up a window that says it has "detected an iPhone in recovery mode."