Watch this video from This Old House to learn how to replace window sash cords.
1 Unscrew the window stop from each side jamb.
2 Remove the bottom sash.
3 Pry the parting bead free of the window frame.
4 Pull out the top sash.
5 Cut sash cord on both sides of window frame; knot cords to prevent them from falling into wall.
6 Unscrew the access panel from the side jambs to reveal void inside wall.
7 Lift out any sash weights that broke free from cords.
8 Attach a length of string to any remaining sash cords.
9 Pull through the access hole the sash weight and old cord, and the attached string and new sash cord.
10 Cut free the sash weight and tie it to new sash cord.
11 Pull new cord and set weight back inside of wall.
12 When there's no existing cord, tie a weight (screw or hex-nut works well) to string and feed it over the pulley and inside of wall.
13 Use string to pull new sash cord into the wall; attach sash weight.
14 Hold top sash in window frame and cut new cords to length.
15 Tie new cord to each side of sash.
16 Install top sash, then replace parting bead.
17 Repeat above procedure for attaching sash weights to bottom sash.
18 Set bottom sash in window frame; reinstall the window stops.