At any time, you can reset an email template back to it's default if there have been changes. To do this, go to Settings > Templates.
Expand the Account Templates dropdown on the Templates screen and find the relevant email. Select the edit button to the right of the template.
The Reset Template button will be on the top right of the screen, click on this.
Note: You will need to confirm this as the action cannot be undone.
To finalise the reset, click on Save Template at the bottom of the screen.