How do you take off the cover of a Dell Dimensions CPU?

IIRC there are two screws holding the shroud.

Remove the screws and gently squeeze the shroud (should help loosen the heat sink) while rotating your hand 90 degrees to the rear of the case.

The shroud is hinged at the top where it meets the case frame at the back.

And if it is the case I remember -- the hardest part was getting the case itself open :) You have to push those releases while lifting and turning the chassis - perfect job for three-armed mutants LOL