As with apps, templates can only be edited by their authors. The predefined templates from Web AppBuilder (Developer Edition) are not editable.
Edit the template
On the home page, click the edit button on the template to edit configurable parameters to the template. When you've finished your edits, save the template and click the Web AppBuilder logo to return to the home page.
Preview the template
On the home page, click the view button on the template to preview it in the browser.