What are “Community Wiki” posts?
I have written an answer to a community wiki question, so my answer automatically became community wiki. Is it possible to convert it to a non-wiki post?
No. It is not possible.
After you make something a community wiki post, you no longer own it and therefore are not able to turn it back to a regular post. This is because you have given your answer to the community, and the community is greedy and won't give it back to you.
(It is also because when something turns community wiki, it is more likely to get upvotes and not be closed as things like subjective and such. This is because Community Wiki posts do not gain or lose reputation.)