That depends on a couple things. First, when you say "tore" does that mean fabric ripped, or a seam came loose. If a seam came loose and you can see how it goes back together, go for it. The other things to consider is your comfort with sewing. If you've put in a zipper before, and feel okay about it, go for it. If, one the other hand, you've ripped fabric and/or feel uncomfortable with sewing zippers, take it to a professional.

okay... if the fabric of the zipper is torn, teeth on the zipper broke or the pull came loose from one side, you will need to replace the whole zipper... this is kind of complicated and requires a zipper foot on your sewing machine... zipper foot looks like half a foot... if you are not a great sewer i'd take them to a tailor... this can be time consuming and frustrating the first time you do it... i've been doing it for 20 years and still hate it (although not as much as button holes .. lol)

if you only popped a seam and you have a sewing machine, run a new seam thru it and you should be fine...