Inside knob should have a small hole at 3 o'clock or 9 o'clock where you can
depress the retainer. (very strong safetypin works well, sometimes small
jewlers screw driver works)
Inside knob slides off.
Big trim plate snaps off to expose mounting screws.
Oh, <slaps head> this is far too dificult for you to do without specialized
tools. You need to call a professional contractor immediately, unless you
want your house full of carbon monoxide, and to burn down in the middle of
the night, killing your whole family. (That's the expected response from
some AHR posters).
Christopher A. Young
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