My iPhone 3GS is running 4.2.1 and I can't seem to update it to iOS 5. Is there anything I can do?

I have downloaded iOS 5 from a seperate site, and frankly I just don't know what to do with it. I also have a first generation iPad which is running on 5.1 so I'm just not quite sure as to what is going on in my laptop (which is an Acer Aspire One netbook, and iTunes is running at 10.1 I think).

Help?

P.S. To be perfectly honest, I'm really wanting this to work now because I want to buy "Zombies, Run!" from the app store and its on sale until this Sunday, and it requires iOS 5. So yeah, my nerdy self is in need of excercise and wants it to be with zombies. Help please?