There are two screws that hold the PCB in place. One is longer than the other. The SHORT one goes to where I am pointing. The long screw holds down a grounding wire. Move the wire out of the way as you take out the board.
There is a ribbon cable that I am pointing at which you can pull out. There are some multi-colored wires that are plugged into the board. Wiggle it out. Move it to the side when you take the board out.
Now lift the PCB out along with the scroll wheel. There should be some more screws which hold the metal to the plastic. Undo those if you want to.
The sides come apart. You must take the mainboard out in order to take the sides off. There are two latches for the left side and four on the right side. The left side also has two screws securing it to the rest of the body.