Technique - Remove a small number of teeth from the base of the zipper to create space to retrack a new slider.

If the slider remains, break it off.

Remove teeth from both sides at the base of the zipper. Take only as many teeth are necessary to retrack the new slider. Singe any fraying.

Put on a new slider where the teeth were removed, catching both sides of the zipper, and slide it up.

Use a top stop or a hand stitch to lock off the bottom of the zipper.