This article explains how to edit the 'Certification Expiry Reminder' email that get's sent to the Certification Expiry contact at the reminder time specified.
1)Navigate to the Certification that you wish to edit the expiry email for.
2) Click 'Edit this version'
3) Scroll down to the 'workflow' part of the email and select 'Send membership reminder to contact prior to expiring'
4) After you have You'll then be able to edit the email text that you're referring to: