Step 1 : Go to  My Dashboard and scroll down to Save the Dates, Invitations & Response Cards, or Thank You Notes.
Step 2: Click on “Select Design” under the “Choose a Design” tab.
Step 3: Explore the available templates. Filter by “Event Type” or “Series” for easier browsing.
Step 4: Select “Choose This Design.”
 
Note: It is also possible to upload a custom template. See the appropriate article here.