When you buy a Leanpub book, you will get all the updates to that book for free. We call an update a new "version" of the book.
You can download the latest version of any book you have bought from your Leanpub library here: https://leanpub.com/user_dashboard/library.
However, sometimes an author publishes a new book on the same topic as the old book. That is called a new "edition" of the old book. In that case, you need to buy the new book.
You can contact the author of any Leanpub book to ask them for clarification on issues like this: