You can. Just slip your left needle into the stitch that's below the first one on the right needle--just put it in the loop. Then slide the stitch off the needle and pull the yarn out. You will have 'unknit' one stitch. Just keep doing that until you get to where you want to be.
For purl rows, slip the needle into the purl bump under the right needle and slide the stitch off.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog