Here:
1) Run Tinyumbrella.
2) Make sure it detects your iphone. Click "Display SHSH." It will disable what firmware you can downgrade to.
3) Download firmware you want to downgrade from somewhere (google).
4) Click on "Start TSS Server."
5) In ITunes, store using file in #3.
6) Should take sometime to restore. If iphone seems to get stuck, in Tinyumbrella, click "Kick Device out of Recovery."