OK, first make sure your phone has at least 50 percent of battery left (plugged in would be even better). Then go to Settings, scroll down to "About Device", select Software Update, and then Check for Updates. If the OTA update is available for your phone, it should pop up there. Just remember, these updates are usually heavy files, so you're better off doing this while on WiFi.
If it won't let you update, then you could try downloading Samsung's Kies (their version of iTunes). After installation, plug your phone to your computer and the software should recognize it. Then, if the update is available, it'll tell you about it and let you download and install it.
If your carrier has not upgraded to Jelly Bean yet, then the update will not be available either way.