Sometimes for whatever reason an iPhone firmware upgrade ends up being counterproductive or causes issues with your individual phone. Maybe you're experiencing battery life problems with iOS 4.1 on your iPhone 3GS. Updates can even happen by accident with the wrong click of a button, wiping a jailbroken device.
If you have already saved your SHSH files for one or more earlier firmware versions on Saurik's server, you can follow these steps to downgrade from iOS 4.1.
First you will need to download these SHSH files to your computer using the free software tool TinyUmbrella.
1. Install TinyUmbrella on your computer.
2. Plug your iPhone into the USB port and run TinyUmbrella.
3. Using TinyUmbrella select Advanced Options and choose the firmware version you wish to run on your iPhone.
4. Select Request Form -> Cydia and click Save My SHSH.
5. Now make sure you have the matching iOS firmware file on your computer. Download the 4.x firmware file for your device here:
Download the 3.x firmware file for your device here:
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6. Put your iPhone into DFU mode.
7. Go back to TinyUmbrella and click Display SHSHs to make sure your SHSH files have been saved on your computer. If the SHSH files are listed click Start TSS Server.
8. Next in iTunes make sure your iPhone is selected and you are in the Summary tab. Hold down the Option (Alt) key on the Mac or Shift key in Windows while clicking Restore. Select the firmware you downloaded from the dialog box.
9. Your iPhone will restore. Errors will likely pop up but they can all be ignored.
10. When the restore finishes select Kick Device Out of Recovery in TinyUmbrella to complete the downgrade process.
Your iPhone is no longer running iOS 4.1.