Hi,

If you are doing it from a custom module, inherit the corresponding form view, and you can change the value of string attribute of the button with name action_create_invoice

1. Inherit the view: purchase.purchase_order_form in custom module

2. Using xpath, change the string of button  action_create_invoice

Sample:

  Certify Billattribute>
xpath>

  Certify Billattribute>
xpath>

 In the given code, some xml tags are missing due to issue in the code editor, please correct the syntax before you try the above code

Thanks

Hi, 
You can create another button with the same name. You can provide any value to the string like string="Certify Bill". Now make the earlier button with a string value as 'Create Bill' invisible using XPath and position="attributes".