Navigate to Setup > General Settings > Invoices tab

Take note of the Store Client Data Snapshot setting:

If the Store Client Data Snapshot option is checked, the client's name and address which was on their profile at the invoice of invoice generation will be fixed on the invoice. Subsequent changes to the client's Profile data will not be reflected on existing invoices.

Unchecking this option will cause all invoices to show the client's current Profile data on invoices.

More information on this feature is located at https://docs.whmcs.com/Invoice_Tab#Store_Client_Data_Snapshot

Navigate to Clients > View/Search Clients and search for the client in question:

Click on the client's name and then select their Profile tab:

Review the Billing Contact setting.

• If the Billing Contact is set to Default, then the client's Profile data will be displayed on invoices.
• If the Billing Contact is set to something else, then the Contact's details will be displayed on invoices

If a Billing Contact is selected, click the Contacts tab

Edit the Billing Contact's details, and this will update the client details displayed on the invoice.

More information on this feature is located at https://docs.whmcs.com/Clients:Contacts_Tab#Billing_Contact