There is a spring and a nut on the end of the stem but inside the casing. It holds and maintains tension with the stem and the casing. To get at that nut you would have to remove the glass mirror part, and I don't know if that is possible without breaking it.
The other way would be to cut a hole in the bottom of the casing which is big enough to get in with a spanner. I have done this on a mirror that was slack to tighten up that nut, but I don't think you will want to go down that route. Maybe time for a can of spray paint instead.
