Run RedSn0w as an administrator. Go to Extras > Even More > Restore. Select IPSW and point it to the IPSW you downloaded for iOS 5.1.1 that matches your device. Then for blobs, if you have them on Cydia, select Remote. If you manually saved them, you'll need to click Local, and point it to the iOS 5.1.1 blobs for your device on your computer. Then select Next. It'll prompt you to choose whether you're aware that the baseband will be upgraded (if you want to preserve your baseband, create a custom IPSW first and use that when selecting the IPSW) and that everything is wiped. You must say yes for it to upgrade. Let it stitch and restore.