You can unvote a post by clicking on the upvote button a second time. Steemit will pop up an alert asking you to confirm your vote. Busy.org will put up the slider bar allowing you to unvote or change the percentage of your previous vote.
It's worth being aware that:
- You do not get your voting power back for that vote, so your voting power (or, put more simply, one of your 10 daily votes) is wasted.
- Removing your vote also sacrifices any curation rewards your would receive on the post, even if you subsequently revote the post at a later time.