Just to help out new users - this is actually easy to do. You can either connect to the database with your favorite sql frontend (I usually use commandline mysql) or you can add the phpmyadmin module to Moodle and do it from Site Administration>Server>Database. Click on certificate_issues and then browse.
Then it's a simple matter of finding the certificate you need to edit and changing their name in the "studentname" field. If you aren't familiar with SQL I would like to put a note of caution - whenever you're messing with the database directly, it's better to backup the database first - that way you can restore it if you accidently break anything.