Carefully insert a thin bladed knife into the gap where the angled glazing bar meets the window frame, at one of the corners, and give it a slight twist. You will find that the glazing bar will spring out of it's register and you will be able to lift the complete bar out. To re-fit the glazing bar, insert one end (with it's profiled locking section tucked into the frame) and tap, with a soft faced hammer, or a hammer and a block of wood, all the way along the bar. I find that a smear of liquid soap, on the locking profile, helps when re-fitting. Also, some light pressure, on the new DG panel, pressing it into the bedding foam tape, helps re-assembly.