Update 3gs to 5.1 jailbroken iphone

I successfully jailbroke my 3gs using redsnow (my first attempt !). Now I see that most of the apps are not supported on the older ios (bummer) I'm using an at&t phone on a straight talk at&t sim card and it all works great after the jail break. I didn't need to unlock anything. I'd like to update the ios to something newer to be able to get the most life out of my phone before having to buy a new one. I don't see any blobs on cydia that show anything newer than 4.1 at the top. I'm a little nervous about doing this. Would anyone be willing to share a good step by step "guide" on how to do this safely ? Thanks in advance.