you must create a new xml file using this template (DO NOT FORGET OWL="1")

<?xml version="1.0" encoding="utf-8" ?>

<templates xml:space="preserve">

    <t t-inherit="addon_name.DemoExample" t-inherit-mode="extension" owl="1">

        <xpath expr="//xpath_expression" position="attributes or replace or after or before">

            <attribute name="field_attribut">value</attribute>

        </xpath>

    </t>

</templates>

And add it in the __manifest__.py file of your custom addon in asset part.