At the moment, on freelance.com, it is not possible for employers to change or delete their primary email address once created.
However, it is possible for you to add this new email address as your secondary email address.
This will allow you to receive all information and communication linked to the alternate email address.
In order to do so, log into your employer account, select the "My profile" tab on your dashboard.
Go to the "Contacts" section (where you will see your primary contact details) and click on the link "Add a new contact" then add your alternative email address.
Once you have confirmed, you will see in this contact section, your primary contact information and your alternative contact information.