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Step One
Right click Notepad from within Start:Programs:Accessories.34d5 26817 How to Downgrade Your iPhone 3GS Using a Cydia Saved SHSH [Windows]

Step Two
Choose Run as administrator from the contextual popup menu.
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Step Three
A popup will appear asking if you would like to give Notepad system access. Click the Yes button.

Step Four
Once Notepad opens select Open… from the File menu.
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Step Five
Choose to view all files from the dropdown then navigate to C:WindowsSystem32driversetc and select thehosts file. Click the Open button.
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Step Six
Append 74.208.10.249 gs.apple.com to end of the file.
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aec2 26823 500 How to Downgrade Your iPhone 3GS Using a Cydia Saved SHSH [Windows]

Step Seven
Select Save from the File menu to commit your changes.
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You are now ready to downgrade your device. Simply put the iPhone into DFU mode and in iTunes Shift+Click the Restore button to select the firmware you would like to downgrade to. If you get error message 1011 or 1013 just ignore it. If you get error message 1015 repeat the entire restore then jailbreak after it fails the second time.