Navigate to the site settings page.

Select a template from the dropdown.

If you want to ensure that a template is used each time a new article is created, you can also check the box to Require that new articles use an article template.

Scroll to the bottom of the page and then click Update.

Also note that whether or not you opted to check the box to Require that new articles use an article template will determine if users can or cannot change the dropdown to "No Template".