You need the remove the plastic beads all around the inside of the window. If it is an old unit, say +30 years then the beads will probably be on the outside; if younger likely on the inside. They are usually very well machined and sometimes difficult to identify.
Then you need a very thin, strong metal plate to insert between the glass and the plastic bead, starting in the middle of one of the longer beads, and carefully pop it out by pulling the plate towards you. They can be very tight. Best wishes for putting things back though; that can be a right pain.
LaurieJB, November 2017
