When you create a Leanpub book, we add a bit of Leanpub-related information to the colophon page (the page after the cover page):
If you don't want to include this information, you can create "unbranded" ebook files for your book.
To do this, go to the Versions page of your book (where you create previews etc.), and select "Unbranded Ebook":
That will take you to a page that looks like this:
To create a version of your book without the Leanpub-related information on the colophon page, click "Create Export":
You will now see download links for your book (and download links for your sample book too, if you have created one):
Here is what the colophon page will look like now:
OK, that's it!
To learn how to customize your copyright information, please see this article.