Motor is on underside of machine. Tilt it back onto it's rear feet and either prop it or have someone hold it for safety.
On the motor you will see two black plastic type units, opposite to each other with wires connected. These are held in place, either by 2 screws or just click in via a tab, and a look at the new one will confirm which type. Once the tab or screws are released/removed, the old brushes will just pull out of the motor. Note which wire goes where, before pulling the two wires off the brush. The new brush will have a sloping end on the carbon tip and make sure you replace the same sloping angle in the corresponding slot.
If the new brushes carbon tip is only partially exposed, a small retaining tab is often present to avoid damage occurring to the new carbon tip, so ensure it's removal to release the carbon brush and it's spring.
Doing one brush at a time makes the replacing job easier and less confusing if doing this for the first time.
Good luck.
Davymac, July 2009