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How to Downgrade iOS 4.1 on iPhone 4, iPhone 3GS & iPhone 3G
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Step 1. Download SHSH Blobs saved on Saurik server, to do so download TinyUmbrella.

Step 2. Now Connect your iPhone and run TinyUmbrella downloaded in step 1. Now check the “Advanced Options” box & under “Device/Version” drop down list select the firmware version to which you wish to downgrade (i.e. iOS 4.0.1).

Finally, select “Cydia” under “Request From”. Then hit “Save My SHSH”. This will grab your SHSH files from Saurik’s servers and save them to your computer.

Step 3. Now download iOS 4.0.1 or the firmware version you selected in above step from here.

Step 4. Put your iPhone in DFU Mode. To do so, launch iTunes and plug your iPhone in. Press and hold the Home and Power button together for 10 seconds. After 10 seconds, release the Power button while still holding the Home button until iTunes pops up and says it has found an iPhone in recovery Mode.

Step 5. Run TinyUmbrella and click the “Display SHSHs” button to make sure your SHSH blobs were downloaded to your computer. Once done, click “Start TSS Server”.

Step 6. Now you need to restore your iPhone for this launch iTunes and plug your iPhone in. Hold the SHIFT key (Windows) or ALT/Option key (Mac) while clicking on Restore. Make sure you hold the key while clicking Restore. A window will open. Simply select the firmware you downloaded in step 3.

Step 7: Your iPhone will be downgraded/restored to the iOS version you chose. iTunes might give you error messages (error 1011, 1013, and 1015). Don’t worry about them.

Step 8: We now need to get your iPhone out of recovery mode. To do so, open TinyUmbrella and click “Kick Device Out of Recovery”.