If you haven't added a delivery method 'Cash on delivery' - add it according to this article.
Go to 'Settings' > 'Payments' > Click '+ Payment method'.
Add the custom name: 'Cash on delivery'.
Change the email template. This message is sent when the order is placed.
Remove the info, for example, about your bank account number and add a few sentences related to the way the customer can pay when the order is delivered.
Finally, connect this type of payment with the appropriate delivery method.