It doesn't seem to have been made available via OTA. If you wanna upgrade you're gonna have to use Kies.
That being said, the benefits of Jelly Bean, while handy, in my opinion are only minor. Camera Zoom, Google Now (which I've rarely found use for myself yet), UI tweaks, ability to use Google Chrome and/or Nova Launcher and tbh that's about it.
You wouldn't get to benefit from Project Butter because the phone is low-to-midrange and not really powerful enough but moreso because Samsung didn't properly optimise the firmware. Whereas the gingerbread firmware has been out for a good amount of time, has been through various revisions and absolutely SCREAMS on the device.
I've tweaked my phone to be more or less as fast on Jelly Bean, but it does require rooting and some specific tweaks/setups.
But ya, if you really want Jelly Bean, use Kies to update as you won't have it available via OTA (unless they've only just released it which I doubt).