Glazing compounds are what hold the glass in place in a window pane. Over time, this glaze wears down, and may even fall off. When this happens, the old glaze will have to be removed, and a new glaze will have to be set in its place. This can be easily done with a few tools and materials from a home improvement store, as well as knowledge of the proper procedure. Removing old glaze will require use of a heat gun, and when you use a heat gun you should always wear a mask, goggles, and leather gloves.
Chip and pry away as much of the old glaze compound as possible with the putty knife.
Soften the remaining glaze with the heat gun, making sure to keep the heat gun moving around the perimeter of the panes. Keeping the heat gun in one place for too long can crack the glass.
Remove the glaze softened by the heat gun with the putty knife
Apply a think bead of glaze around the entire pane where the glass meets the frame. Run the putty knife around the frame of the window to remove any excess glaze. Allow the glaze to set in and harden. Repeat these steps for all individual window panes.