I accidentally down voted an answer (fat fingers on a small phone) and didn't notice until after it was locked. How can I undo my down vote?
Votes can only be undone when the post is edited. If your vote really was accidental, you can resort to editing the post and then changing the vote.
However, try and make the edits substantial — especially if you don't have edit privileges yet, your edit might be rejected for being too minor. In case of doubt ask the OP in a comment to edit their own post or ask a user with 2k reputation to do the edit for you (e.g. through chat).