JB iPhone 3GS can I update firmware?

My iPhone 3GS is jail broken and running 4.3.3 firmware. I would like to use it to stream to my TV but I can't install skygo due to the old firmware. Is there an 'easy' way to upgrade it to 5.1 or something so that skygo will work on it? If that is possible would I be able to stream to an Apple TV and run my XBMC through it?

Sorry if these are dumb questions but you don't know what you don't know!

Thanks