I use a small blade screwdriver. Place the blade between the metal bezel and the plastic body and twist, slowly opening up the metal bezel until you can lift it up off the plastic lip of the bucket and remove. If I recall you have to open a bit more than half of the circumference.....enough to get it off. Slow and steady she goes.
Once the fix is complete, replacing screws, cleaning the lens, repairing the needle etc. I get the face back on and place the speedo down on a soft but sturdy surface and gently tap the bezel crimp back into place with a tack hammer. There's a thread somewhere with the info in this forum.