How to repair seam on leggings

If I had access to a sewing machine I would double stitch it, the principle for hand repair is the same.

Choose a similar colour polyester thread stitch over on the same spot to secure thread using small stitches, sew a row of running stitches where you see the original stitching would have been. When you get to one end turn around and resew the same line carefully sewing in between along the first row you sewed, if some seams have started coming undone, it's important to do the whole seam as it is likely the rest break. Don't forget to stitch over in the same spot to finish.