You can view and modify your Client Invoices as System Message Templates.

The following templates are relevant:

• Client Invoice
• Client Invoice Payment Receipt
• Client Invoice Overdue Notice

You'll find the place to do this in your My System >> System Message Templates.

One part of Client Invoice templates is the Invoice Header, given by the [invoiceHeader] magic tag.  That, too, is customizable.

You'll find the place to do so in Business Center >> Setup >> Invoicing Config >> Miscellaneous >> Invoice Header Extra.