To change the vials in the 42-321, 42-331, 42-347, 42-449and 42-350 wood levels, you would pry out the outside edge of the rubber bezel. The window will pop out with it. When it's open, you will see a metal plate with notches. The vials are set in these notches. Knock the old vials out of the notches (there may be slight adhesive on there, but just scrape that off). Press the new vials into the notches. You may have to turn them slightly to adjust. Press the rubber bezel and window back on.