A separated zipper can make getting in and out of your clothes a struggle. Instead of ripping the old one out and sewing on a new one, try these steps to get your zipper back in working order. See How to Fix a Stuck Zipper and How to Unjam a Sleeping-Bag Zipper for more zipper tips.
With a small pair of pliers (needle-nose pliers work best), pry off the zipper's metal stop, the little band at the bottom of the zipper that stops the slider when you open the zipper (see A). If you don't have needle-nose pliers, you might be able to pry off the stop with a screwdriver or even a blunt kitchen knife.
Once you've removed the stop, move the slider all the way to the bottom of the zipper, just below the bottom-most teeth.
Realign the zipper's teeth so that they mesh smoothly.
Move the slider up to the top of the zipper, zipping it completely.
Thread a needle with strong thread. Tie a knot at the end of the thread.
Starting with your thread on the back side of the zipper, make six or seven stitches across the bottom of the zipper where the old stop was, creating a new stop out of thread (see B).
With your thread on the back side of the zipper, tie a knot in the thread and snip it off with your scissors.
If the zipper is merely sticky and not separated, try rubbing a candle along the teeth of the zipper, then zipping and unzipping it several times. This quick fix will often make the zipper work more smoothly.