I downvoted a question but later changed my mind. What do I do? Your help would be appreciated!
Downvotes and upvotes are locked for 5 minutes, see Why do votes get locked? for the reason why:
You have a short period of time to change your mind on a vote, five minutes. After that, you can only change your vote if the question or answer has been edited.
This helps to prevent irregularities in voting. If I could go back and take away every up vote I ever made, more than a few people would see a 1k + drop in reputation. If you thought the post was good, or not good for the duration of the grace period, the system assumes you knew what you were doing when voting either way.
You can only retract your vote once the post has been edited. So you might want to suggest an edit to improve the question, and retract your vote after the edit has been approved.