As I recall, you need to soften that rubber tape that holds the metal to the glass, then ease the metal off without bending it out of shape. A guy I used to know had a glass shop, and he would lay the window with the metal on his work table, squirt some alcohol from a bottle on around the edge of the metal/glass interface and light it; when it burned out, he just pulled the metal off. Today, I would think a heat gun would be the better tool for that. Putting it back is pretty much the same in reverse. You need to soften the "rubber" tape to get the glass to slip into it.

.