There are 4 screws that must be removed to allow access to the mouse's internals. These screws are located under the rubber pads on the bottom of the mouse, which can be carefully removed by pushing a small flat head screwdriver underneath.
There are no screws located under the right-most pad (closest to the thumb wheel).
Take care when separating the base of the mouse from the outer shell: the two are connected by a ribbon cable. This cable can be disconnected safely but is long enough to allow full access to the inside of the mouse while still connected.