Requirement to downgrade your iPhone 4S, iPad 3 or iPad 2:
You cannot downgrade without the personalized SHSH blobs for your device at that lower firmware. You need to have fetched those blobs while the signing window was open, using either Cydia’s built-in TSS@Home feature, or with TinyUmbrella. The new Restore screen of redsn0w lets you choose either the remote blobs or local ones (for the earlier firmware). If you don’t know where TinyUmbrella put your blobs, TinyUmbrella has a button that will show you (copy them out of that folder and feed them to redsn0w).
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