The reason I ask is because mail doesn't work with CNAME records!
Mail operates using MX records!
In fact, the RFC strictly prohibits the use of CNAME with mail servers.
So if you are talking about email in any fashion (and it looks like you are)
then you are working with the wrong DNS record entry!
Editing the zone is the same in either case and has to either be done
from WHM or manually from the shell.
From WHM: Go to "Edit DNS Zone" under "DNS Functions"
From SSH: Open the file /var/named/domain.db
(where 'domain' is your domain name)
If you don't have either WHM or root SSH access then you will need to contact
your hosting provider to have them update this item for you.