First of all, I would suggest a better quality speaker, like alpine. The sony Xplodes look cool, but the quality of them are not too good. In order to gain access to the factory speakers, you need to remove the door panel. Usually there is one or two screws that will need to be removed. These are usually behind the door handle or in the arm rest. Once those are removed, the entire panel is held in place by clips. You will need to start at one point on the door, and use flat head screwdriver to pop out the clips from the door. Repeat this around the entire panel. Once all clips are out the panel should come right off. You will then have access to the speaker.
Are you going to be adding an amplifier along with these speakers?
Let me know how it goes.