If a missing or broken zipper teeth at the very top or bottom, you can sew a new zipper stop above the missing teeth to prevent the slider from moving past the gap. If the missing or broken teeth ...

To do this, bring the slider to the top of the zipper. You can use a seam ripper to remove fabric stitching if needed. Make sure you remove the zipper stop, if there is one, by cutting it with pliers.