If you require a new letterhead, BE Software will need to be engaged to do this work.
If however, you need to make small changes to an existing letterhead, you can try the following:
From the Options Menu, under Invoice Letterheads, click on Edit.
Provide a new name for the new letterhead and click on the HTML icon displayed. If the right-hand side of the menu is not appearing, cancel and retry.
Select the text displayed using the mouse to copy it via "Ctrl C" and then click on Cancel to return to the previous screen. Then click on Cancel again to return to the main screen.
Select the Invoice Letterheads tab click on +New, and then provide a new name for the letterhead.
Click on the HTML icon again, and paste via "Ctrl V".
Make any modifications to this new letterhead such as an ABN change. Then Save to complete the process.
Click on Close to return to the main Menu.
Ensure you select your new invoice letterhead template in Case List > Case tab so it will apply to any invoices you generate: