Cut any paint applied over the interior stops with a utility knife. Stick a stiff putty knife between the stops and the outer frame and pry the stops off.
Lift the window up to expose the metal track on each side. Pry the metal track off with a putty knife.
Remove the window from the frame.
Pry off the parting strip -- a thin piece of wood or vinyl -- on each side of the window with the putty knife.
Remove the weight pocket cover. Pull out the weight and remove the weight from the sash cord.
Measure the old sash cord and cut a new piece the same length. Slide the new cord into the pulley and attach the new counterweight to the cord. Replace the window.