Just go to WHM, List Accounts. You can update the contact email for each account here.
You can only send WHM notices for a cPanel to one email. This should be the reseller owner, as each cPanel user can set their own contact email in cPanel for cPanel notices.
Is there a way to change the email address from which the contact email is sent?
No, this will always email from the server.
VPS / Hybrid / Dedicated Clients
You can have the WHM notices sent to more than one email using the Contact Management link in WHM.
