You take the entire laptop apart, then put the new processor in the old one's place. With laptops you can only purchase a few different versions of the processor you already have (usually the ones that they already offered when purchasing the laptop).
I know with other compaq's you take it apart, then get to the heatsink and take that off, under that is the processor, you take that out and replace it with the new processor (obviously must be the same socket and it would be wise to stick with the same model otherwise you will run into overheating).