Hello Jakub
You can change the report string with help of following code.

<record id="stock.action_report_delivery" model="ir.actions.report">
    <field name="name">Delivery Slip Changed</field>
</record>

Hi,

Please try the code to change your string,

<report string="Delivery Slip"
id="stock.action_report_delivery"
model="stock.picking"
report_type="qweb-pdf"
name="stock.report_deliveryslip"
file="stock.report_deliveryslip"/>    

Thanks

Muhammed Ali M - iWesabe

Hello Jakub Patch,