You should be able to download the latest version of the book from your Leanpub library, if you are signed in to Leanpub: https://leanpub.com/user_dashboard/library.
If you need to reset your password to sign in to Leanpub, you can do that here: https://leanpub.com/password_reset.
If you can find the download email we sent to you when you made the purchase, the download links in that email will always point to the latest version of the book, so you can use those download links to get the latest version of the book.
If you don't see the book in your Leanpub library, you may be signed in to the wrong Leanpub account.
If you discover you have accidentally created more than one Leanpub account, here are some instructions for merging accounts: