That would mean finding that vote and changing it through the vote lines in the server's script. Sure it's possible, but do we really want people changing it to be annoying? If I voted 10 times on 10 different saves, Up, then they do something bad, does that mean I can change it all to vote downs?
What if I genuinely make a mistake on a vote and vote it down and the person hates me for that and they never talk to me again, but I really meant to vote up? I should have a way to correct my vote and vote the person up. Of course this should not be abused. It should be used to change a vote if an honest mistake was made the first time. But you should NOT deprive me or anyone else of the right to get to change our vote if we really DID click the wrong button the first time.
You need to think about the people who do abuse it. We can't see votes that were mistakes, we can't decide "oh this one SEEMS like a mistake" and change it. You need to think of what someone else can see.
Well is there some way to make it like, one vote change a day? That would mean you can only mess up once a day, and if you have made another mistake after that than that's just too bad