Click on the 'Achievements & Progress' button in the green box on the left hand side under the title of the course.
This will open up a side menu, click on 'Change Name'
Alternatively, you may also see this side bar:
Clicking the above options -- "Change Name" or "Request a Name Change" -- will prompt you to check the box next to 'I am not a robot'. Do so, then click the 'Send Security Code' button at the bottom.
A security code will then be sent to the email associated with your account. When you receive the security code enter into the appropriate box.
Once you submit that security code you will then be able to change the name on your certificate.
Please note* It may take a day for the change to take effect. Check back on your certificate 24 hours after changing, if it is still not changed AFTER 24 hours you may email into firstname.lastname@example.org for extra assistance.