unpublishing a story

Quite some time ago, I started a story called "the secret of M.Y.T.H.", and had high hopes for it. unfortunately I got blocked on the story, and it fell to the bottom of my stories in progress pile. Now, at last, I think I can do something with it, but rather than simply carrying on from the point where it has been sitting, I think it would be better to unpublish the unfinished version and then publish the complete one. So how can I remove the old version of the story?