The thermal pad should come off in one piece. After it is off, use a Phillips screwdriver to remove the five screws holding the heat sink in place.

Once the screws are removed, it will be easy to separate the heat sink from the PCB. You will also notice a second thermal pad located under the heat sink. You may remove this pad as well.

Once the pads are removed, they may either be reattached to a new heat sink or new thermal pads may be placed on the new heatsink.