If you update iPhone 4 to iOS 4.2.1 then your baseband will also upgrade to 03.10.01 and at the moment you can’t unlock this baseband. Here is the solution, how to update iPhone 4 to iOS 4.2.1 without updating its baseband. We can achieve that using TinyUmbrella. Lets see how to preserve iPhone 4 baseband 01.59.00 using TinyUmbrella while updating iOS 4.2.1.
Note: Use this guide to update iOS 4.2.1 on iPhone 4 only.
Connect iPhone 4 with computer
Run TinyUmbrella and hit the “Save SHSH” button,
See the SHSH for iPhone 4 4.2.1 in the list on TinyUmbrella
Now hit the “Start TSS Server” button and keep it running
Update iOS 4.2.1 iPhone 4
Now its time to update iPhone 4 to iOS 4.2.1.
Note: Before updating iOS make sure there is no error occurred in TinyUmbrella, click on Log tab, if TSS Server is running then you are good to move on the next step.
Run iTunes, Select your device from the left panel.
Shift key from keyboard on Windows or
Option key on Mac and then press the “Restore” button in iTunes.
Now browse and select the iOS 4.2.1 firmware file for iPhone 4 that you have downloaded in step-1
You will see this 1013 error – baseband update failed, this is what we are doing.
You have successfully update iPhone 4 to iOS 4.2.1 without updating its baseband.
Now if you want to jailbreak iOS 4.2.1 check this: How to Jailbreak iOS 4.2.1 iPhone 4 Untethered