Adding a centered subtitle to main title

How do I add a subtitle to my novel’s main title so that it’s centered? In other words, I want to add the subtitle UNDER the main title. Also, the main title is currently written in block letters. How do I change it so only the first letter of each word is capitalized?

The answer came from Jeff in Support. I was using Metadata to produce my title. But now I understand I can simply replace the Title placeholder with whatever title and subtitle I want. Problems solved!