It's pretty simple when you finally get it right. This is how you do it:
Remove the screws from the bottom. Take the amp onto your lap, front facing away from you. Take a good hold of the trannys, tilt the amp a bit (lift the back of the amp) inside the headshell. Now just drop the headshell leaving the amp in your hands. (ie. hold the amp faceplate facing the ground, the headshell should just drop off pretty easily).
Of course before this can work you'll need to remove the screws from the faceplate and the wooden blocks from the back of the amp.