Updating a certificate, that is already being used in a course, can be reflected to your users with two ways:
a. As Instructor enter to the course and click on the Users tab (1).
There you will see a list of the users assigned to the course. Go next to the name of the learner you want to update and click on the certification icon (2).
The certificate will display and by clicking on the green "Update" button you will synchronize the specific template with the changes or you can update all the certificates for this course (3).
b. As Administrator go to the User Reports and after you select your learner move to the Certifications tab (4).
Click on the relative icon (5) for the certificate to pop up.
Click on the green "Update" button (6) and changes will be applied directly.