I found that my answer for a question someone asked is inaccurate. I would like to down vote my answer, partly because its not a good answer and partly because I want to move it as low as possible in the list of answers.
Can I please know why I am not allowed to do this?
The best thing to do would be to edit your answer to make it correct. Baring that (or if someone else already has the "best" answer and you don't think you can do better), you can do as @dnbrv suggested and delete your answer.
You can delete your answer, which will hide it from the public view but you can't vote on your own posts.
