To apply a template or copy another mail shot design click the 'Apply template or copy another design' link as shown in the image below.
User Template takes you to a list of your templates.
From the list that appears, click on the name of a template to overwrite your existing design with a template.
You can copy the design of another of your mail shots by following the above instructions and then clicking the Copy a Mailshot tab from the dialog that loads. The image below shows this:
Stock Templates
You can use and adapt one of the Stock Templates by selecting from the list.