Andy is technically correct as usual, but it doesn't always work out that easy in practice. You'll want to remove the instrument from the dash first, which can be an adventure in itself if those oddball mounting threads are tight. Then between the metal ring and the glass is a layer of some kind of fiber/rubber material that may crumble and fall out. If so, you'll need to replace it with something that is thick enough to hold the glass securely, but not too thick to install the ring.
Then putting the ring back on can be somewhat frustrating too. You have to hold the instrument face up, so gravity will hold the glass in place, while you simultaneously get all 8 or 10 tabs engaged and turn the ring to lock them. Can be a problem, especially if you have small hands.
I'm not saying "don't do it", only that it might be a little harder than Andy makes it sound.
Oh yeah, best to work over a towel or something, to cushion and catch the glass if you happen to drop it. The edges are surprisingly fragile.