I would imagine it is just glued in like the newer square mirrors are, just use a gasket scraper and pick and pry away at it, make damn sure to wear safety glasses cuz a lotta small pieces are gonna be flying around,
Once you have the assembly reconditioned and ready for new glass(most glass shops will be able to cut this for you) re glue it in using silicone only (lots) in my experience any glue stronger than this, such as windshield urethane will eventually peel the silvering off the back and you will see marks in the pattern of which you applied the glue in a couple of years time.

Mikie
near Ottawa Canada
64 econo p/u & 58 Merc 100 panel