Turn off all electricity to the unit and disconnect the motor wires.
Loosen the allen screw that attaches the motor coupling to the gearbox shaft. Remove the 3 bolts that hold the motor cover to the fan housing. Pull straight out. If you have the old style coupler, just remove the 3 bolts and pull straight out. If the cover won't pull straight out, gently use a chisel to move the cover away from the fan housing.