How to downgrade iOS 4.2.1 to 4.1 / 4.0.2 / 4.0.1 for iPhone 4, 3GS, 3G, iPod touch, and iPad

Historical text from HellasProject (2009–2014). It is not current advice or a present-day service.

iOS 4.2.1 came out yesterday for iPhone 4, 3GS, 3G, iPod touch 4G, 3G, 2G, and iPad. Even an iOS 4.2.1 jailbreak was achieved with Redsn0w 0.9.6b2, but Cydia still does not run (though with a manual patch something happens). Worse, it includes a new baseband for all iOS devices. If you flash that you will not be able to unlock the modem with the current ultrasn0w, so if your device is from the US or carrier-locked, stay away. For everyone else who is factory unlocked, bought in Greece, went up to iOS 4.2.1, and wants to go back to iOS 4.1 / 4.0.2 / 4.0.1, follow the instructions at the link below to downgrade iOS 4.2 on iPhone 4, 3GS, 3G, iPod touch, and iPad to a previous version such as iOS 4.1 / 4.0.x.

More in the article they prepared for this job at redmondpie.com.