If payments have been applied to the invoice, you won't be able to change the customer in the Customer field. You need to delete the payments on that invoice and then edit the invoice to enter a new customer. See Deleting a customer payment.

To edit an invoice

You can click invoices to open them for editing.

1. In the invoices list, find the invoice you want to edit and click it. The Invoice page appears, showing the invoice details.
2. Make any changes to the invoice. You can make any changes you like to the invoice, such as add or remove line items, change the due date, even change the customer.
3. To save the changes without sending a copy to the customer, click Save. The Invoice page reappears.
4. To save the changes and send a copy to the customer, click Email or print or Resend to customer (the option that appears depends on whether you've previously sent the invoice). In the window that appears, complete the fields and click Email or Print. For example, if you're resending an invoice previously sent, you might want to update the Subject and Message:

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For more information, see Emailing, previewing and printing invoices.