There is a couple ways of handling this. If its a fairly simple pattern you can do like Sue suggested and *tink back a row stitch by stitch. Or you can sometimes just knit two stitches together (k2tog). If you have two extra stitches k2tog twice near the beginning and end.
*tink - literally knit backwards, but it means to unknit stitch by stitch. Another term is to frog or frogging (think of a frog sound "ribbit" which sounds like "rip it.
There's a video in tips/fixing mistakes of tinking I think.