To copy of a certificate and create another certificate type from an existing certificate, first select the certificate in the Home Screen, then click on the Create Certificate button in the ribbon and select the option you want:
If you are creating a new Minor Works from an existing certificate, you will be asked to select a board and circuit from the source certificate. The details of the circuit you select will be carried forward into the new Minor Works.
You can also merge boards and observations from other certificates into an existing certificate. Read about that feature here.