The clip under the cup is shaped like an upside down split "T" (or two "L"s back to back and joined at the top), you need to squeeze the near side in-toward the other and then push it up and away, once the clip has cleared the hole. The other clip then comes right out. I usually do it by hand, but in some cases I've used a pair of needle nose pliers. You may need to lift the edges of the red suction cup to determine the orientation of the clip so you know which direction to sqeeze it.
Fortunately the SE/30 has a bleeder-type resistor so discharge should be unnecessary. But Tom Lee's method is worth doing just in case Murphy's Law rears its ugly head.