To remove the pump,

1. Take off the skimmer cup.
2. Loosen the screws that hold the plate sitting behind the pump.
3. Remove the sliding plate by sliding upwards out of the body.
4. Using the mains cable carefully pull back on the pump until the suction pipe clears the intake side of the protein skimmer and carefully lift out.

When re-installing the pump make sure that the plate that sits behind it is all the way down and locked into place via the screws. If the plate is loose it will vibrate on the pump and make a humming noise.