You'll see your certificate when you finish your course, and you can grab it to download or print again at any time. Here's how:
Sign in to your account here.
Once you've logged in, on your main page, click on "My courses" in the top right.
This page shows all of your courses, click the drop down menu and select "Completed"
Find your course, and click on "View certificate"
Select "DOWNLOAD" or "PRINT" at the top of your screen.