Removing side mirror to fix

My work truck is an '01 Silverado 1500. Recently, while pushing the passenger side mirror back to the "open" position the clip or ring that holds the mirror tight to the post on which it sits popped off. I am trying to pry off the plastic shroud around the mirror but it will not budge. My mirrors have the chrome covering attached to the plastic. Does anyone know how to remove the plastic covering so I can push the ring back on the post to tighten the mirror?

Thanks

Jeremy