You can use a program called Beehind to downgrade your device. But you have to install OpenSSH in Cydia first before you downgrade with this tool.
Heres a link to the software. A video tutorial is provided with this link:
http://geeksn0w.it/Beehind/
NOTE: This tool is Windows only. This software is fairly new, so an update for a Mac version may be released soon.