<p>If you realize your mistake immediately you should be able to undo an upvote by clicking upvote a second time (at least it works for me); similarly for a downvote. (Try it on my answer here -- click upvote, say "oops, didn't mean that", then click <em>up</em> a second time and the vote should come back off again. Then click upvote a third time because <em>my answer is just the greatest, we have the best answers, everyone thinks so</em>.)</p>

<p>It may seem a little counterintuitive at first but once you get used to it, it will seem fairly natural.</p>

<p>If you don't realize you want to change your vote fairly quickly (more precisely, within <a href="https://meta.stackexchange.com/questions/5212/what-are-the-limits-on-how-i-can-cast-change-and-retract-votes/5213#5213">five minutes</a>), you'll be outside the window in which you can change your mind, in which case your vote will not be changeable until the post is edited.</p>
