Self Assessment dashboard >> General settings >> Click on the email template that needs to be edited and customise accordingly 

Video Tutorial:

Click here to watch how to set up email templates in Self Assessment

Help Guide: 

From the Self Assessment dashboard, use the navigation panel to go into General Settings >> Email Template 

Select the template you want to change and click on Edit. 

To use the template go into a clients dashboard and using the action section from a tax return that has been generated, select Email.