I just voted mistakenly up-voted an answer, then tried to cancel it by clicking on the down arrow. Of course, that changed my a down vote, which wasn't I wanted even less. So I changed it back to an up vote, which is no big deal. I'm wondering, however, if there's a way to just cancel the vote entirely.
Once you vote on a question or answer you have 5 minutes to undo or change your vote. You can undo an upvote by clicking on the upvote arrow again and likewise with downvotes. After 5 minutes your vote is locked and you can neither undo nor change your vote. However, any edit done to the post removes this lock and you can then undo or change your vote. If you vote again, either by undoing and redoing or by changing your vote, a new 5 minute window starts.