The process is usually as simple as buying a new map DVD and loading it into the appropriate Navi slot--just like changing a CD. The map updates usually cost $300. My father-in-law's Prius has it's nav system DVD slot under the driver's seat. I found the location by poking around in the suspplemental navigation system owner's manual he had in his glove box.
Getting a new disc is something the part department at a Lexus dealer can help you with. Once you have the new disc and know where the navigation system's DVD slot is, installing it is as easy as chaging a CD.
Read the following thread in the Lexus owner's forum.