Downgrade iPhone 3G-3GS IOS 4.1 to 4.0

For various reasons, we may want to go back after the 4.1 firmware
to switch back to 4.0.Stuff you will need1. iTunes 10 (latest
iTunes)2. iOS 4.0 Firmware3. iRecovery (Mac)4. LibUSB Library Package
(Mac)5. iRrecovery & LibUSB Library Package (Windows)Step 1: Download
and put OS 4.0 on your desktop, now put your iPhone into DFU mode by
following the instructions.* Connect your iPhone to your computer.
*
Turn off the iPhone.
* Press the Power button and Home for 10 seconds
(exactly).
* Release the power button but hold the Home button until
the computer beeps (observed on a PC) as when a USB device is
recognized.
* A few seconds later iTunes should detect your iPhone.
*
If the logo of this restaurant is on the screen, you're in recovery
mode, not DFU.Step 2: Once your iPhone is in DFU mode, the following
message shall appear in iTunes.Click "OK", now hold the Option key
(Mac) and Shift key (Windows) by clicking Restore in iTunes and find
the firmware of your choice by browsing the OS 3.. 1.3 / 3/.1.2
firmware for your iPhone 3G or 3G.Now, the restore process will take
10 to 15 minutes.Step 3: After the restore process if you have
encountered an error like 1011, 1013, 1015, so just ignore them (in
fact the firmware is already successfully installed).To exit recovery
mode Recboot download once you complete your download Recboot, run it
and click on the output mode of recovery.