Thanks to iFaith you can downgrade iOS 6.1.3 without SHSH blobs to 5.1.1 or with locally saved blobs by Tinyumbrella to 6.1.2. As for me I don’t have blobs from TinyUmbrella so I used that iFaith gave me. Last time I wrote about how to dump SHSH blobs and create custom firmware using iFaith. But here is a little different method and I tried it myself on 3GS.
Just few minutes ago iH8sn0w released new version 1.5.6 to dump SHSH blobs and create custom IPSW and you download iFaith for Windows for free using direct link below. The new update came to fix iOS 6.1.3 downgrading with right blobs for it. It appears that Cydia isn’t saving your iOS 6.1.2 SHSH blobs correctly on Saurik’s server. If you are planning to upgrade or downgrade iOS 6.1.3 to 5.1.1 or other version using iFaith, you have to read this article now.