Look down the two notches in the top of the collar & you should see a tab (on the collar) in a notch (on the casing) down each one. Pry the collar away from the casing at the notches. If you have a drum brake adjustment tool (the one that looks like a bent-up double-ended flat-tipped screwdriver), or some other bent flat-tip screwdriver, this shouldn't be too hard. Otherwise, you should be able to remove it with a regular flat screwriver, as long as you remember that you're prying on plastic parts with a steel tool.
Ken, June 2007