Sash windows are designed to be easily stripped for repairs.
If the window is painted use a utility knife to cut through the paint seal around the stop bead and pocket pieces, following the gap.
Force a paint scraper into the gap between the stop bead and the jamb, and another scraper further up to prise the bead free.
FREE THE LOWER marking the position of the old cord on the stile then swinging the lower sash free from the frame.
Disconnect the cords and lower the sash weights to the base of the jamb.