How to update SSL certificate when moving webmail from roundcube to the new webmail interface

Hi

How does one renew the SSL certificate after changing the CNAME from server.mxrouting.net to webmail.mydomain to access the new webmail interface?

When I go to Advanced / SSL certificate, it shows that it's already been issued and that the DNS is incorrect because it expects webmail to still point to server.mxrouting.net.

Edit: or are you not supposed to point webmail.mydomain to webmail.mxroute.com?