Whether you have a self signed certificate or a commercially available certificate, changing it is very easy on a Mac Mini.
1- Simply go to Server Admin and under the servers menu your server should be highlighted.
2- Select the certificates tab at the top and the current certificates will be listed.
3- Click the plus sign to create either a new self-signed certificate or import a private key and public certificate you own.
4- The minus sign allows you to remove a expired certificate already on the server.