If you can find the extra YO, you can let it slide off your needle with no ill effect. If you've already worked it somewhere, then you can do what Suzeeq said.
If you haven't done anything to that row yet, look at the stitches on the needle. You should have the pattern going of the first k, then a yo and the k2tog. They look different, so you should be able to tell which one doesn't belong. If it's just a yo, let it slide off the tip of the needle. It will seem loose there for bit, but it will even out.