From HEXONET Wiki
The RenewSSLCert command can be used to renew an existing (probably expiring) certificate. The same CSR data will be used to create the new certificate.
IMPORTANT: Please have a look at the SSLCERT API_manual for more details.
COMMAND
(required) command = RenewSSLCert sslcertid = (SSLCERTID) period = (PERIOD)