The dome and plastic valve above it. You can get to the valve from above, couple of pipes going to it, and there is a retaining clip molded into it to hold it onto the pipe of the dome but it should pop off easy enough. The dome is more difficult to remove, it is a pressed fit into the block and best accessed from underneath. Use a small wedge or screwdriver all around the lip to gently work it free. Try not to damage the mating face of the block.
I personally choose to replace the dome because of the press fit, but they are around £30 from VW so others have re-fitted them seemingly without problems.
