Zippers are very useful pieces of hardware---they are great closures for everything from jeans and backpacks to some shoes and camping tents. However, over time and with lots of use, it is possible for the zipper's slider to come off its track. When this happens, the zipper cannot zip up. In the event that your zippered item's slider comes off its track, replace the slider to avoid having to replace the entire zipper or buy a new item.
Cut the end stop off the new zipper and remove the slider from the zipper. Make sure you use a zipper that is the exact same size and teeth count as the old zipper so the size of the slider is correct. It's recommended that you use a new slider, even if you have the old one that jumped the track. This is because the old one could be worn or broken, which is why it came off track in the first place.
Push the slider onto the top, right side of the zipper you are trying to repair. Simply push the zipper onto the right-hand side of the slider. It might need a bit of a forceful push to get the slider on track.
Bring the newly installed slider down to the bottom of the item, where it will be held in place by the end stop of the old zipper. It is now ready to be zipped up, as it normally would.