With nine screws, three on the top and three down either side, on very early ones there are two on the bottom but their rare.
Often their covered in mastic so hard to see so make sure you count them before trying to move the heater.
The mastic will often also hold it tight so some force will be needed to break the seal, I use two 1/2inch chisels and a thin piece of wood to protect the van and give some leverage, start at the bottom and work your way up. If the mastic is very strong a Stanley knife dipped in white spirit is useful to cut through it once a slight gap is made