In Australia we have an ABN (Australian Business Number) for tax purposes and so I have entered this into the VAT/Tax Number field however now on the invoices it has prefixed this with the text VAT/TAx Number. How can I change this text to read ABN?
You can go to Event Espresso -> Messages -> Default message templates -> Invoice.
The [INVOICE_PAYEE_TAX_NUMBER_*] shortcode has a prefix attribute you can use to change the text, so change that shortcode to something like:
The output would then be ABN:1234
Just to note, you can find details on all of the shortcodes available in the current template within the help section at the top of the page when editing the template. The shortcodes are all listed under Help -> Message shortcodes, with some help text for each one.