There will be two black wires connecting the top mounting bracket to the reverb box at the bottom of the unit. Label the wires to prevent crossing upon re-installation, then unplug the wires from the reverb box.

Modification: If there are not two black wires and a reverb box, there will be two twisted pair of wires attached to the board coming from a spring reverb mechanism on the right side of the case. You will need to spread out the clip and pull these wire connectors from the board. Be sure to record which color goes where!