3GS iphone with version 3.1.3 - I want to upgrade the ISO

I have a 3GS Iphone with the original version 3.1.3 system on it and firmware 05.12.01. I would like to upgrade it. I had to update to itunes 11 and I am just getting the option of iso 6 when I select the update. It takes forever for anything to happen so I have cancelled it. Is there any tips or help someone could give me. Thanks.