You can get the certificate by going to your course Tracks at https://leanpub.com/user_dashboard/sets, and then clicking on the course Track, and then the "View Progress" button.
After that, you will see a "Complete Course Track" button. Clicking on that will allow you to get your certificate.
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Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book