How can I change the standard message sent with a docusign please?

Without knowing the exact information you are trying to change I can only be vague in my answer. You can change the Email Body and Email Subject with each envelope. Also you can set a standard email subject and email body within a Template so that you do not have to change the email subject and body each time you create a new envelope (use the Template instead). There are also ways to further modify the email text by using the Email Resource file which is outlined further in the following document...

https://support.docusign.com/en/guides/docusign-signing-resource-file-v2-0