Did mine recently. The inner parts of the cap are pressed into the outer chromed cap with a flat square spring steel holding the whole works in place. If you pry the corners up a bit at a time with a screwdriver you can work the innards up and out. Really hard to explain but if you look closely you can see it. Once the new gasket is on you just press the assembly back in with your fingers. It's good to clean everything up when it's out, there was quite a bit of crap in mine and it won't vent properly if it's plugged up.
Sorry if this isn't clear, I'm not great at explaining things, but I'm sure someone will pipe in.
Don
