Thank you for joining the thread, @S2 Accounting.

I'm happy to share the steps on how to update the primary and secondary contact of your customers in QuickBooks Desktop.

To update the primary and secondary information:

1. Select Customer Center from the Customers menu.
2. Click on the customer's name under Customer & Jobs. 
3. Tap on Contacts in between Transactions and To Do's under Customer information.
4. Right-click on the primary/secondary contact, then click on Edit Selected Contact.
5. Update the information from there.
6. Hit Save and close to record the changes.
7. Repeat the steps for the rest of the information you want to change.

I've added these screenshots for your visual reference.

Post again if you have any other questions. I'm always here to help. Have a good day!