I don't believe you can retract that approval-vote.
What are the limits on how I can cast, change, and retract votes? doesn't say anything about edit-approval votes, and neither can any other meta.SE questions about "edit" "approve" "vote" and "retract," "revoke," "rescind," or "take-backsies."
But you can revert the edit.
At the bottom of the post should be a clickable link: "edited [$TIME] ago." Clicking there brings you to the post's revision history.
Look to the most-recent version of the post you like--in this case probably the second-to-last post. Click "rollback."
I don't believe there's a way to leave an edit-comment on a rollback, so I suggest you leave one on the main post explaining why you object to the edit. (That way if there ever is some reason to dig through the post's revision history others aren't left wondering why this one reversion happened.)