To change your vote on a film, simply vote again. The new vote will replace the previous one.
To delete a vote just go to your Vote History page. Find the vote that you'd like to delete or change, then hover over the gold star bar. If you want to change your vote, pick a new star rating and click. If you'd like to delete your vote, hover over the starbar and look for the blue "x". Click on the "x" to remove your vote.
It may take 24 hours for your change to be reflected on the title's