To access all certificates you have been issued, visit the My Courses page and click "All Certificates":
Here you can access your most recent certificate for each course, and even see previously issued certificates.
If instead you want to just see certificates for a specific course, find that course in the list and choose "Certificates":
Finally, if you wish to see the certificates for a specific completion, simply click on the course name to expand it and then choose the "Certificate" button next to the relevant completion:
