If an Employee's certification for a standard is set to expire, you'll receive a notification.

To sign them up to be re-certified, navigate to the "Employee Certifications" tab (on the right) - you'll be shown a full list of your Employees and their current certifications. 

Locate, by searching or filtering, the employee and certification in question, and select the "Re-Certify" button on the left. This will bring you to a list of Course Offerings available for that specific standard.