Generally there are only two main places where one inputs an email address with the My Cloud. First is under the Dashboard > Users section for each User. Second is when one creates their MyCloud.com online web portal account. As far as I know there is no official way to change the wording of the WD email message that gets generated when one uses the email link feature. A third place where an email address is used is the alert email notifications under Dashboard > Settings > Notifications.
The workaround to change the email address is to change the User email address within the My Cloud Dashboard and (I assume) then create a new MyCloud.com web portal account using that new email address.
The inability to change the MyCloud.com email address has been complained about before.
https://community.wd.com/t/request-for-change-of-email-address/144214
https://community.wd.com/t/change-e-mail-adress-of-wd-community-account-not-possible/45576
