The Template widget inserts a custom global template into your page.
From the Content > Template tab > Choose Template, Select a template that you’ve previously saved in Templates.
After the page is published, an Edit Template button will become available in the widget options. If you need to edit the template, click the Edit Template button to quickly navigate to the template within the Template Library 
Note: Editing and updating the template in the Template Library will cause all uses of that template widget across the site to update as well.