An email template can be deleted for many reasons, like if it has never been used, will never be used again or if a newer version has been created.

To delete saved custom templates:

1. Prepare to send an email

2. After filling out Email information, go to Design tab

3. Click Select a template drop-down

4. Select All custom templates 
User-added image

5. Choose unwanted templates

6. Click Delete
User-added image
7. Click Yes to confirm
User-added image

Note: Default Email Templates can't be deleted