If the selected customer is a Company, you can easily maintain an up-to-date list of their contacts right at the Contacts tab on the full inspector and use those contacts to quickly update the customer's primary contact, billing contact, alternate contact, or the site contact whenever required.

Once you have the details of the contacts in place, you can set them as a Primary or Billing contact.

To know more about the types of customer contacts in detail, see Introduction to Company Contacts.