like you say this should be easy, just take off the rack ear reinforcers and remove the obvious screws on the upper back panel above the connectors, on the sides and on top behind the front panel.
The lid/top of unit just slides off over the back connectors and away from the front in one piece.
Maybe yours has become jammed, ease it off gently resting the back panel on your knee (don't drop it!) making sure that both sides are sliding down at the same time, it sticks if you pull one side too far ahead of the other. sometimes it needs a gentle knock or tug to get it moving.
There's no catch or anything!
Hope that helps