Since there has been some confusion from people about Kies, the BIG problem that some people have(not just here on this thread) is that you need to be using Samsung Kies. Not Kies Air or something else. Samsung Kies is a really badly written app that is somewhat similar in function to Apple iTunes, but for Samsung devices. You can download it here:
Kies - Sync Media, Contacts & Calendars Across All Devices | Samsung
Kies itself has been known to have issues, but is the best, and to my knowledge only way to install the official JB update(you don't need to root or do anything fancy).
So, get Kies, install it on your computer(must be an administrator account on Windows, not standard or limited user), and then plug your phone into the computer. You will see drivers get installed if you are on Windows. Once the drivers finish installing, Kies itself should now show you what firmware you have on your phone, and will now offer the update.