How to use a saved SHSH blob to downgrade firmware: http://stateofjailbreak.com/tutorials/how-to-downgrade-firmware-using-tinyumbrella/
In this video I show you how to save your SHSH blobs on your iPhone or iPod touch using iSHSHit. iSHSHit is a free download that allows you to send your SHSH to Cydia as well as email a copy to back up locally on your computer.