> Where do I have to put this? In the exiting file in
> trytond/modules/account_invoice/invoice.xml? Or should I create a new
> module?
It's a custom development = new module.
> And for the company/header_A4.odt: Should I just replace this file?
Not necessary
Example to replace default sale report
<record model="ir.action.report" id="sale.report_sale">
<field name="name">Sale</field>
<field name="model">sale.sale</field>
<field name="report_name">sale.sale</field>
<field name="report">yourmodule/new_sale_report.odt</field>
<field name="style"></field>
</record>
