2. Start Tiny Umbrella with your iPhone 4 plugged in. Save your SHSH files for iOS 4.2.1. (You should be saving your SHSH files whenever a new firmware comes out!) UPDATE: Apple has stopped signing SHSH blobs for 4.2.1, so if you do not have them saved, you can’t update to 4.2.1. I will release a tutorial on how to update to 4.3 soon.
3. Click on Start TSS Server. If you get any errors, visit the TinyUmbrella FAQ page and fix the errors before continuing.
4. Launch iTunes and click on restore while holding down the shift key if you are on windows or the option key if you are on Mac.
5. Select the iPhone 4 iOS 4.2.1 firmware file. If you dont have it you can download it here: iPhone3,1_4.2.1_8C148_Restore.ipsw (If it saves as a .zip file make sure to rename it to .ipsw).
6. Once it is done restoring you should get a 1013 error. Click on OK and close out of iTunes. Your iPhone 4 will have the connect to iTunes logo displaying. If you get an error other than 1013, it means your baseband was updated. This is likely because you did not have the TSS server running or you are using a device other than an iPhone 4.
7. We need to get the iPhone 4 into DFU mode by holding the home and power button until the screen goes black. Then count to 2 and let go of the power button but keep holding down the home button. Your screen will turn black if you did it correctly. If not you will see an Apple logo and the screen will go back to the connect to iTunes logo. If you see this you need to start step 7 over.
8. Once you are in DFU mode start GreenPoison and follow Tutorial: Untether iOS 4.2.1 Jailbreak to jailbreak your iPhone 4 while preserving your baseband.
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