A4 Devices running iOS 6 have already been jailbroken even still tethered. Somehow you may prefer to downgrade it back to iOS 5.1.1 because that version is so far the best version with untethered jailbreak. To downgrade, you will need the latest public version of RedSn0w. We will use the SHSH Stitching feature to combine the appropriate IPSW file for 5.1.1 and your corresponding SHSH blob for the device you wish to downgrade.
Steps To Downgrade iOS 6 to iOS 5.1.1
Download RedSn0w 0.9.14b2 here
Download the iOS 5.1.1 IPSW file for the corresponding device you’d like to downgrade.
Locate the SHSH blob you saved earlier. You must have an iOS 5.1.1 SHSH blobs saved for you device, and stored on your local computer, or on Cydia server. If you have it on your computer, locate it, and place it in the same folder as the 5.1.1 IPSW file. If it’s stored on Cydia, then continue on.
Place your device running iOS 6 into DFU mode.
Fire up RedSn0w, and click Extras >> SHSH blobs >> Stitch
Click IPSW and select the iOS 5.1.1 IPSW for your device.
If your SHSH blobs are stored on Cydia, Click Cydia under the blobs section. If you have your SHSH blobs stored locally, click Local, and select the SHSH blob for iOS 5.1.1 for your device.
Once the IPSW and blob are selected, RedSn0w will stitch them together to create an all encompassing firmware file for iOS 5.1.1. You will be able to use this firmware file to downgrade, even though Apple is no longer signing iOS 5.1.1.
In RedSn0w, go back two pages, and click Pwned DFU. This will place your device into Pwned DFU mode so that iTunes can restore using the stitched firmware. Without doing this, you will get error 1600 in iTunes.
Open iTunes, and it should detect your device in recovery mode. Hold Option and click Restore, then select your newly created stitched firmware file. The filename should be prefixed by a series of 13 numbers, which is the device’s ECID
Allow iTunes to completely restore your device, and you may be presented with an Error -1 from iTunes. Do not worry.
Open RedSn0w again, and click Extras >> Recovery fix.
Place your device back into DFU mode, and the Recovery fix should run successfully.
Once the Recovery fix is completed, your device will be completely restored back on iOS 5.1.1.
Here’s the video walktrhough in case you are still confused..