How to replace a zipper
A. Purchase a new zipper from a fabric shop of the same length as the broken zipper. While you are there buy some strong thread of the same
colour to match
B. Rip out the broken zipper carefully using a seam ripper. You might have to open up a little bit on the inside of the waist band.
C. Set up your sewing machine with the above thread. close the new zipper to make it easier to attach to the trousers.
D. Pin the new zipper on to the underneath piece of the zipper opening. Tuck the top of the zipper into the opening of the waist band. Ensure your new stiches are exactly where the old stitching was.
E. Pin the zipper to the top panel and stitch where the old stitch was