First, note that most of the time when Leanpub authors click the button to publish their books, they're publishing a new "version" of an existing book, not a new "edition" of the book.
New versions of a Leanpub book are always free for people who have bought the book, and they are automatically available to everyone, including new readers who buy the book for the first time.
A new "edition" is a new, different book.
The main reason to create a new "edition" is when you have essentially written a new book on the same topic.
New editions typically only happen every couple of years.
If you would like to publish a new version of an old Leanpub book, please see this article.
If you would like to offer readers of the old edition a discount for the new edition (people really appreciate things like that!), you can do that by following these instructions.