Just a simple tutorial on downgrading your iDevice.
Note.
*If you don't have SHSH blobs saved, you won't be able to downgrade.*
SHSH blobs are basically what Apple uses to sign the firmware. Apple won't sign a firmware that is out of date, to slow down jailbreaking. So the SHSH blobs sign the firmware, so you can install a lower firmware than what you're currently on. You need to have jailbroken your iDevice on that specific firmware, and saved the SHSH, if you want to downgrade to it. So if you're on iOS 5, and you've never jailbroken your iDevice before, you might as well leave this thread. However, if you are on 4.3.5, and want to downgrade but didn't save your SHSH blobs, there is supposedly a way to do that, HERE. If anyone can confirm that that works, please do so.

What you will need.
-iDevice
-Computer
-Firmware you wish to downgrade to
-SHSH Blobs for that firmware
-TinyUmbrella (Windows . OS X )

Steps

1. Go ahead and grab the firmware you'll be downgrading to:
http://www.felixbruns.de/iPod/firmware/
Find the firmware for your device, and download it. I recommend doing this first, because it can take a somewhat long time on slower internet connections.
2. Put your device in recovery mode.


Hold down the power and home buttons for 10 seconds, letting go of the lock button but continuing to hold menu until iTunes recognizes the device with the message: "iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes.".
Note that, at this point, your iPhone's screen should be entirely black. Many people confuse "DFU" with "recovery" mode, but it's not. iTunes itself says it is in recovery mode.


3. Now iTunes should say your device is in recovery mode, that's fine. Just leave it.
-If your device screen isn't black, then you didn't do something right. Repeat step two until your device screen is completely black.

4. Now we will need to set up a server, to stop iTunes from verifying the restore with Apple. Download TinyUmbrella, from the links i posted above. Hint: In the Materials list.
Run tiny umbrella (on Windows right-click, then choose run as administrator). Tiny umbrella will not show your SHSH automatically, you have to download them from cydia first. To do this click "Save my SHSH", this will download all available SHSH for your device from cydia, if you have saved them. If not, you shouldn't be this far in the tutorial.

To restore, click Start TSS Server in TinyUmbrella.


Now open iTunes, and option-click on the restore button, for the iDevice you put into recovery mode. Browse for the firmware you downloaded, and select it.

Now iTunes should begin restoring your iDevice to the firmware you selected. TinyUmbrella will redirect iTunes, to stop it from verifying the restore with Apple.

Now wait for your device to finish, and you will be on the firmware you selected.

Restoring removes everything on your iDevice, so backup before you start.

Any questions, problems, concerns, let me know.