Getting A/C plastic lens out to clean

[Spiderwolf]

My wife just bought an 05 Cabrio S. On the dash, or actually below it where the Air conditioning controls are, you have a round knob you turn to turn up or down the temperature. There is a round clear piece of plastic protecting the temerature readout. Someone at sometime must have spilled what looks like coffee on the controls. There is a small amount of a dried substance that looks like it could be coffee on the inside of the lens. Im curious how this comes apart so we can clean the inside of the lens. Does the know just pull off? I wanted to ask before I start pulling on it and break something. Maybe someone here has had to do the same thing at some point or another.