The sealed units in a window are held in by the glazing beads which need to be carefully prised out with a sharp chisle or scraper. Locate the joint between bead and frame and push in the blade in the middle od the length of bead, twist the blade and the bead will rise. Push a matchstick or small screwdriver into the gap to keep it open then twist the blade a little further along. You should then be able to pop the bead out. Once one is out the rest are relatively easy. Replacement is done inreverse but smear a little fairy liquid onto the rubber to slide the bead into the mitre of the corners.