At the edge of the plastic housing you will see the lip of a brass holder. Pull this out using fine tweezers or wire cutters (just to grip, not to cut) . From the other side of the brass holder you can pull out the 8x5mm carbon brush.
You can pick up a replacement set for less than £2 at amazon. You will need a dremel tool or similar to make the slot on two sides of the replacement brush so it matches the original, or you could cut back the brass a bit so it doesn't need slots.
Mark Taylor
April 2013