If you are wanting to downgrade the firmware on your iPhone, iPod touch or iPad it is possible using TinyUmbrella. Before you begin however make sure that you have previously saved your SHSH blobs using TinyUmbrella (a tutorial on how to do this is available here) with the SHSH blob for the firmware you would like to downgrade to already saved.
With TinyUmbrella open click the Start TSS Server button. It will close iTunes if it is open so make sure you aren’t doing anything important.
Now open up iTunes and connect your device. You can now restore to any firmware that you have a SHSH blob saved for.
Hold the Shift key and left click Restore (Option + Restore on a Mac) to select what firmware you would like to downgrade to. If you don’t know where the firmware you are looking for is click here for a tutorial.
After you point iTunes toward the firmware you are basically done, just sit back and let iTunes restore your device. Congratulations, you have now downgraded the firmware on your device!