To edit a certificate that has already been awarded to some of the learners in your system, log in as an administrator. 

From your Administration dashboard then visit the Certificates section and then move on to the Users tab (1) where you can find all users that have been awarded a certificate.

Then click on the edit icon to edit a certificate (2) that has been awarded to a learner for a specific course. A pop up will appear with all the tagged values in your certificate template along with their current values where you can edit each value.