Hi.....the top is fixed but the front panel moves side to side. I'm a bit wary of moving the front too much in case I dislodge or break whatever is behind.
Usually the panel with the button will pull forward, you may need something to give it a little nudge past the spring clips as it is hard to get your hands in.
Try pressing the button partially in and often you can use that finger to pull the panel off by applying sideways and forward pressure in the hole cutout.
What’s next to it? If you really struggle I have done it before by drilling a small hole diagonally through from the inside of the next unit and poking a long thin screwdriver through to force the wc panel off from behind.