How can I upvote/undo downvote of a question/answer/comment which I mistakenly downvoted? Similar is the query about a mistakenly upvoted question.
Find a way to improve it. After you edited the post, you can change the vote.
If there is no way for that, then bookmark the post and watch, maybe others will later edit it.
If there is no change, then your possibilities were depleted. However, the system is driven by the stats and not by the induvidual votes. Thus, if your votes are based on careful consideration, it helps a lot more, as some rare mistake harms.