You can do this on the "About" page for the book, which is in the book's "Settings" section:

You can get to this page by going here:

https://leanpub.com/YOUR_BOOK/settings/about

...making sure to replace YOUR_BOOK with your book's unique web address.

On that page, you will see a section where you can enter your Table of Contents as HTML:

Did this answer your question?