Hi Ian - all good advice. Until I can get my hands on one of these "can openers" a small screwdriver will suffice. Fortunately the "steel ring" as you describe it, I call it a bezel, is thin gauge metal and is easily bent. Just work with the screwdriver around the bezel where it contacts onto the housing and pry the metal away in small increments. Eventually that area will be straight enough to remove the bezel and glass. To replace simply use pliers to bent the metal back around the instrument.
Bob