The only way to get the Jelly Bean upgrade is directly from Samsung. It must be installed using a Windows or Apple PC. The update is not expected to be available as an over the air download directly to the phone.
If you have Samsung's Kies installed, connect your phone to your PC with a USB cable and launch Kies. When Kies starts it should notify you that the update is available and prompt you to install it.
If you don't have Kies, it's available here for Windows and Mac OS. You can also download the update bundled together with Kies from Samsung's support pages: Virgin Mobile S II support page, Boost Mobile Galaxy S II support page.
Here's what's new in Jelly Bean for the S II:
"Project Butter" performance and stability improvements, Google Now, new TouchWiz Nature UX user interface from the Galaxy S III, Samsung’s S Cloud services, improved camera controls, a new lockscreen, Samsung's new Direct Call, Smart Stay and Pop-up Play features, some extra notification toggles, two Home screen modes and some new widgets.