To delete a message template, go to 'Templates' from the Dashboard. From the list of Templates, click 'Delete' on the one(s) you no longer want.
To edit the template you will want to select the template you would like to edit from this list in the left panel.
You will now see the right panel open up with your template's title and text box open. You can now edit your template and click "Save Message" to keep your changes.