In the settings menu you can under "About phone> System updates" as far as firmware goes
if you're talking about the PRL, it's *228 (*ACT, short for activate since they share the same service) and hit 2 when the chicks voice comes up and it will update the PRL information on your device. http://www.phonescoop.com/glossary/term.php?gid=11 in case you have no idea what the PRL is, it basically is how your phone connects to the towers around you. It can be helpful to update it once in a while as it can sometimes help your phone identify new towers in the area giving you better reception.
But other than that there's no dial-code for updating... at least none that I ever heard of... and I lurk forums pretty hardcore :\
Edit: PRL DOES NOT APPLY TO ANY 4G PHONE... *disclaimer*