Remove four (some only have three) jewelers screws on the back of the mounting flange, and the faceplate comes off with the glass (note how the knob mechanism interfaces). Clean the glass (vinegar/water and newspaper works great on glass) and flow a dab of superglue between the edge of the glass and faceplate in several places. Reassemble and good as new!
On installed instruments, a small suction cup will help you hold the glass in place while you flow the superglue with a toothpick.
