Today I voted up the wrong answer. I noticed that there's no way to undo your votes. When you have voted an answer you are forced to vote it, even if you don't want to. I immediately noticed the error and I could just change my vote to up or down, not delete my vote. It makes no sense at all, imho. Since you are able to change your vote within 5 minutes, you should also be able to remove your vote entirely. I can't believe no one noticed that before...
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You can cancel it: click on the vote you've cast again, i.e. click on the up arrow again to cancel an upvote.
As you've noticed with up / down you can only do this for five minutes. I did exactly the same as you and ended up downvoting to cancel at first. At least I got the critic badge out of it :-)