iOS 4.1 beta was released late last night exclusively for the developers and we even posted the guide to jailbreak iOS 4.1 with redsn0w 0.9.5 beta 5 on iPhone 3G and iPod touch 2G (non-MC models). In case you feel you have made a mistake upgrading to iOS 4.1 beta and want to downgrade it to iOS 4.0, here is a guide to do that.
But keep in mind that by downgrading iOS 4.1 to 4.0, your baseband will remain the same and your iPhone will be locked if you are not on supported carrier. Hence unlockers must stay away from iOS 4.1 until iPhone dev team comes up with iOS 4.1 unlock tools.
1. Download iOS 4.0 firmware file.
3. Start iTunes 9.2 and Windows (Alt Key+ Click Restore) Mac (Hold Option + Click Restore), Now manually select iOS 4.0 firmware.
4. During the restore process you will get an 1015 error. Don’t worry, that’s normal. Just click on Ok and close iTunes.
7. Now unplug your iPhone and turn it off by pressing the home+power button.
8. Turn it on now by pressing the power button. Plug-in your iDevice and load iTunes to continue with activation.
Your iPhone 3G should now be downgraded to iOS 4.0. iPhone 3GS users can downgrade to iOS 4.0 only if you had saved your ECID files of earlier version.