Hello,

Your code will change it in order_line form view but if you want to change it for list view use the below:

I used [@name='product_uom'][2] because there are two fields with name product_uom

<record id="view_purchase_order_form" model="ir.ui.view">
	​<field name="name">purchase.order.form</field>
          <field name="model">purchase.order</field>	​	​
          <field name="inherit_id" ref="purchase.purchase_order_form"/>
          <field name="arch" type="xml">
                 <xpath expr="//field[@name='order_line']//list//field[@name='product_uom'][2]" position="attributes">
                    <attribute name="string">Mengeneinheit</attribute>
                </xpath>                   
            </field>
</record>

Result will look like: