RSA server certificate CommonName (CN) 'www.mydomain.com' does NOT match server name

The SSL cert purchased was for www.mydomain.com. Server name, under the IP:443 container, is mydomain.com.

If I try to Virtualmin>Server Configuration>Change Domain Name to add the www before the domain name I get "Failed to change domain name : The domain name does not need to have www at the start"

How do I keep this warning from showing up in my Apache files?

RSA server certificate CommonName (CN) 'www.mydomain.com' does NOT match server name

Is manually editing httpd.conf a solution? Is that the only solution?

Thanks