You have the renew option at the bottom of the certificates page :
certificate renew optionIn case a certificate expires or is about to expire, select the corresponding certificate and click Renew. Set the validity period (in days) for the certificate and click OK. If the firewall is the CA that issued the certificate, the firewall replaces it with a new certificate that has a different serial number but the same attributes as the old certificate. If an external certificate authority (CA) signed the certificate and the firewall uses the Online Certificate Status Protocol (OCSP) to verify certificate revocation status, the firewall uses the OCSP responder information to update the certificate status.