How to Update iPhone 4 to iOS 4.2.1 While Preserving the Unlockable Baseband 01.59.00

A new way has been discovered to upgrade iPhone 4 with the baseband 01.59.00 to iOS 4.2.1 while preserving the baseband, which is unlockable using Ultrasn0w 1.2.

This method will be useful for those who are on iOS 4.0.2 with Baseband 01.59.00, as they can preserve baseband 01.59.00 and can use Ultrasn0w 1.0-1 to unlock iPhone 4.

The below steps will guide you to update iPhone 4 to iOS 4.2.1 while preserving the same older baseband 01.59.00 and unlock it. Make sure you backup everything on your device before procedure to update. Please proceed at your own risk

First download the iOS 4.2.1 firmware ISPW files.

• iPhone 4 iOS 4.2.1 firmware file

1: Go to Start > Programs > Accessories, and Right click on Notepad, than choose Run as administrator from the contextual popup menu.

2: In Notepad, select Open from the File menu then navigate to C:WindowsSystem32driversetc and open the hosts file.

3: Now add 74.208.10.249 gs.apple.com to end of the file and save it. This is done to trick the Apple server into thinking that the iTunes is watching Apple servers, while in reality it watches Saurik’s Cydia servers.

4: Now, you’ll have to restore your iPhone 4 to iOS 4.2.1 by clicking on the iPhone icon on the left sidebar of iTunes then hold left “Shift” key (on Windows) and Alt key (on Mac) and press the “Restore” button. Choose the original iOS 4.2.1 firmware and click on Open.

5: Now, just sit back and relax while iTunes does the work. iTunes should pop-up with some error as error 1013, 1015 or some other error. Ignore it, you are done now.

6: Now, since your iPhone will be stuck in recovery mode, you’ll have to get it out of that mode. So download TinyUmbrella (Windows – Mac), run it and click on the “Exit Recovery Button”.

Go to Settings > General > About and check iOS & baseband version. Your iPhone 4 should be on iOS 4.2.1 with Baseband 01.59.00.

You’ll now be able to jailbreak your iPhone 4 using Redsn0w 0.9.6 for tethered and Redsn0w 0.9.7 for untethered one, and then unlock it using Ultrasn0whow carrier unlock.

Stay tuned with us on Twitter or join Facebook Fanpage, we will keep you updated on such iPhone, iPod Touch and iPad Jailbreak news.