Hi ,
Refer link : http://www.cisco.com/c/en/us/support/docs/security/asa-firepower-services/118824-configure-firepower-00.html
You will first need to shutdown the CX card and then install the Firepower cards on it , the procedure is explained on the link above. Is the CX up and running ?
Regards,
Aastha Bhardwaj
Rate of that helps!!!