To change the title of your book, go here in the Leanpub menu:
Author > Books > [Your Book Title] > Settings > About
You can also go here:
https://leanpub.com/YOUR_BOOK/settings/about
...making sure to replace "YOUR_BOOK" with your book's unique web address.
You can also find this option on your book's Overview page under "Settings" here:
The "Overview" page for every book is here:
https://leanpub.com/YOUR_BOOK/overview
...making sure to replace "YOUR_BOOK" with your book's unique web address.
If your book is published, you will want to publish a new version of your book, after you change the title.