1. Start by removing the trim around the window. Score the edges of the trim with a utility knife to loosen the paint, then gently pry off the trim. (Not sure how to do this? Watch this video.)
2. Remove the sash. The cord may be broken on only one side, but it’s best to replace both while you have the window open.
3. Hold the cord to gently lower the weight, then cut and remove the visible sash cord.
4. Access the weight, pulley, and the rest of the sash cord by removing the panel cover or window casing in the jamb. Use a screwdriver to pull out any screws and gently pry off the panel cover. (Need help locating these parts? Watch this video.)
5. Remove any remaining pieces of sash cord. Clean and spray the pulley with a lubricant.
6. Run a new cord through the pulley. Use a ¼"- to 5/8"-thick cotton cord with a nylon core.
7. Tie the cord to the weight through the hole at its top. Place the weight in the jamb and test it by pulling it up and down. Pull the weight to the top of the window, then run the cord through the channel on the sash. Cut the cord 3" past the hole at the end of the channel and knot it.
8. Replace the panel cover, then reinstall the sash—top one first, followed by the bottom. Put the trim and the parting strips back on with 3d finishing nails, roofing nails, or short screws (not deeper than the wood). Fill any nail holes with spackle or compound, and touch up the paint.