Jelly Bean for Samsung S III is out, all you need to do is go to your Settings -> About -> Software Update and if that did not work for you then just download Samsung Kies for from here (Windows) or (MAC) to get started.
Once installed and your device is recognized and correctly connected through the USB, you should first back up your phone just in case of any failure, which i did and it was just very easy thing to do.
Just click on the Back up/Restore on the right up side and select all the data you need to back up from your device to your PC and wait til its completed.
After finishing, a pop up window will show up and ask you to upgrade to the new firmware and all that is left is to accept and wait for it to download. 😀
and here it is Tadaaaa!