Hello - I'm running version 15.5 on Debian. My godaddy cert will expire in a couple of weeks, but I can't seem to figure out where to upload the new cert from.
in linux you can use the openssl command to generate your CSR
in your godaddy portal you'll have to rekey the cert and paste the CSR into the portal. once they generate the cert then you'll be able to download it
as far as what to do from here once you have your certificate, i'm not too sure about debian. i've installed certs on various versions of linux but not with 3cx. perhaps someone else can add more detail here.