There is two ways to change an email message of a purchase order, atuomatically and manually.
Automatically
Go to Purchae Order Settings.PDF
Click on Email Message then change the informations based on your needs then click on Save.email
Modified email will appear at every purchase orders when you will send the order by email.
Manually
To change only one email of a purchase order, follow next steps:
Go to Purchase Orders submenu from Orders & Deliveries module and click on an order.mark
Click on Send by Email (or click on Actions then choose Send by Email for Sent and Received status orders).emailsend
Change the email based on your needs then click on Send by Email.email