You can 'unknit' your stitches, one at a time. If it's a knit, put your left needle into the stitch below the one that needs to come off the right needle and gently pull your yarn out. For purls, put your needle under the purl bump on the row below. As you do it a couple of times, you'll see exactly where to take it out. Try to have your yarn in the front or back depending on which kind of stitch it is. If you don't, it's no big deal, you'll just have to slip the stitch, move the yarn and slip it back.