After watching iPhone Firmware 3.0 preview I am sure many users would have attempted to download leaked iPhone Firmware 3.0 and upgraded their iPhone 3G to firmware 3.0. But one thing you should know before attempting to do this is that you need to have your iPhone UUID registered with apple to try Beta Firmware 3.0 and its illegal to download leaked version of Firmware 3.0
Anyways if you have already ended up upgrading your iPhone 3G firmware 3.0 and feeling to downgrade back to Firmware 2.2.1 then follow below guide, however you must know you can only downgrade your iPhone Firmware 3.0 but not baseband 04.20.01.
How to Downgrade iPhone Firmware 3.0 to 2.2.1
1. Connect your iPhone to your Mac or PC and select your iPhone from the list of devices in iTunes
2. If iTunes does not detect your iPhone then you will need to put it in DFU (Device Firmware Upgrade) mode.
3. Browse the Summary Tab,
Hold down Option and press the “Restore” button
Hold down Shift and press the Restore button
4. Now browse the folder where you have the iPhone firmware.Select the firmware & hit the restore button and cross your fingers for 5 minutes.
5. Ignore error message like 1011, 1013, 1015 (Actually the firmware 2.2.1 is already successfully installed)
Steps to put iPhone in DFU mode
1. Connect your iPhone to your computer.
2. Turn iPhone off.
3. Hold power and home together for 10 seconds (exactly).
4. Release power but keep holding home until the computers beeps (observed on a PC) as a USB device is recognized.
5. A few seconds later iTunes should detect your iPhone.
Please Note :- If you encountered with error message like 1601, 1602, 1600 or anything else then try putting the
iPhone into DFU mode and try again.