How do I set a title as fiction or non-fiction

I notice on my statistics page that 115 of my books are listed as 'not set' on the pie chart showing fiction or non-fiction. How do I set this status? I've tried a couple of obvious fixes but so far no luck. How would you go about this?

On each book, you'll see a spot in the right hand margin that says "Genres" and a pencil. If you click on that, it can set the Fiction/Non-fiction notation.

Yes, that did the trick. Thanks very much indeed.