I just recived this replacement housing and cap as well.

I was able to get the old one off by first removing the screw, then prying with two screwdrivers from opposite sides between the old cap and the base.

Mine popped right off but it required me to pry from both sides at the same time.

When doing it this way, it didn't require very much force, that was my only concern as I didn't want to break anything.