How To Update iPhone 4 to iOS 4.2.1 While Preserving 01.59.00 Baseband[Windows/Mac]

Step 1:

Download the original iOS 4.2.1 firmware for iPhone 4 from here.

Step 2:

Now trick iTunes into thinking that it is watching Apple servers, while in real, we are going to change the address to Saurik’s server. To do this:

Navigate to C:\Windows\System32\drivers\etc\ and locate “hosts” file.

Mac OS X users can get to “/etc/” directory by going through “Go –> Go to Folder” from Finder.

Open this “hosts” file in Notepad (for Windows) or TextEdit (for Mac OS X) and add the following line at the end :

74.208.105.171 gs.apple.com

Now simply save this file and you are done.

NOTE for Windows 7 and Vista : Start “Notepad” with Administrator privileges by right clicking on “Notepad” app, and click on “Run as Administrator“ option.

NOTE for Mac OS X: Make sure you have full read and write access on your account when you are saving the host file. If it still throws permission error, simply copy the host file to desktop, make the required changes, and then replace it with the original file in “/etc/” directory.

Step 3:

Start iTunes, click on your iOS device icon from the sidebar in iTunes. Now press and hold left “alt” (option) button on Mac, or Left “Shift” button if you are on Windows on the keyboard and then click on “Restore” (Not “Update” or “Check for Update”) button in the iTunes and then release this button.

This will make iTunes prompt you to select the location for your iOS 4.2.1 firmware file. Select this ipsw file and click on “Open” to restore your iPhone.

Step 4:

Now at some point while restoring to iOS 4.2.1, iTunes will throw 1013 error at you. This is normal, just ignore this error as the firmware has already been successfully installed on the device without the upgraded baseband.

Step 5:

Your iPhone at this point will be stuck in Recovery Mode, showing “Connect to iTunes” screen.

To exit your device from Recovery Mode, download a program called TinyUmbrella. Run it and the click on “Exit Recovery” button. Your device will now restart normally.

Once you have completed this procedure, Go to Settings>General>About on your iPhone, and scroll down to check your Modem Firmware (Baseband), which should now read 01.59.00 (which of course is unlockable using Ultrasn0w).

You should now be able to jailbreak your iPhone 4 on iOS 4.2.1 (tethered) using Redsn0w 0.9.6, or untethered using Redsn0w 0.9.7. Once you have jailbroken your iPhone 4, you can safely unlock it on iOS 4.2.1 (on 01.59.00 baseband) using Ultrasn0w 1.2