This is straight forward. There are four bolts that hold the top and bottom together and you need to start off by removing these. Once they are out of the way it just leaves you with the problem of separating them.
Due to units being fanless they rely on using heat-conductive pads to connect any heat-generating ICs to the finned metal of the top cover. These use some form of adhesive on both surfaces to help with the heat transfer.
The one with the SiS 550 CPU was fitted with a heatsink with a large square pad on top of this. This was very firmly attached to the heatsink and the top cover. Even with the application of a considerable amount of force it didn't want to move. (At one point I wondered if I might be in danger of ripping the CPU from the board but luckily the heatsink is fixed to the board so I think that was taking the strain).
The trick was to slide a small flat-bladed screwdriver (or similar) through the gap between the side and the top. As soon as I got this between the pad and the top the two just came apart.
The others used significantly smaller rectangular foam pads and came apart more easily.