If you sign in to Leanpub and go to your library here https://leanpub.com/user_dashboard/library, you should be able to select the book, and then see a "Download Extras" link, which you can click to download the files.
Here is an example of a Leanpub book selected in the library, with the Extras download link highlighted:
If you're new to Leanpub, here's how this works: we automatically create an account for you when you make your first purchase. This is done so that you can always access the latest version of your purchases (Leanpub books and courses can be updated at any time). If you've never set a password for your account, you can do this here: https://leanpub.com/password_reset. Once you sign in, you'll be able to go to your library to download all of your purchases.