I upvoted an answer by mistake. I wanted to downvoted it. I get "You can't change your upvote without edit the answer" or something like this.
How do I deal with it?
Forget about it. A single vote on the wrong direction — whatever direction that may be — will probably not affect anyone very much. It happens; move on.
If you really want to change (or rescind) your vote, though, you'll be able to do so once the post you voted on has been edited. So keep checking back on it for edits and then you can vote again. Alternatively, if there's something that you see you can really improve in the post such that editing it is worthwhile (honestly, not just to allow you to vote), go ahead and edit.