The old IGU will be stuck with sealant. Wear gloves—broken glass is razor sharp. Carefully lift the old unit out. Clean the frame channels completely, removing all old sealant and debris.
Most modern windows have vinyl, aluminum, or wood stops (beads) holding the glass. Pry these off carefully with a putty knife. Label each piece (top/bottom/left/right) as they look identical but rarely are. double pane window repair