In reply to alexm198:
Use backstitch and practice on a scrap of fabric first.
Cut the stitches holding the old zip. Try not to cut the fabric.
Measure and buy a new zip.
Use pins to get the zip where you want it. If it doesn't look right, move it around until it does.
Tack the zip into the right place using bright coloured thread so you can see the stitching. Do not rush these 2 stages. Check that the zip opens and closes ok.
When you are happy with the position of the zip, use backstitch and matching thread to sew it in place. Normally zips are sewn in when closed, but this probably won't be possible with a bag.
Remove your tacking thread carefully.
Pictures to follow....
Post edited at 11:34