This is my son's favorite backpack, so even though it is wearing out, I decided to replace the zipper. First, measure the length needed and buy one as close to it as possible. I used one I already had on hand. Sometimes you can find a heavy duty one at the store. On another backpack, I used two zippers and had them meet in the middle. Next, pick out all the stitches to remove the old zipper. Notice how the original one was positioned. You will want to put the new one in the same way. Just pin in the new one and stitch, using a zipper foot. This simple repair can save a lot of money!