A quick look in the Dreamweaver Help files using the word TEMPLATE for an index search gives you full instructions as follows;
To edit a template file:
1. Choose Window > Assets, then click the Templates icon to view templates.
The bottom of the Assets panel lists all of the available templates for your site. The top area displays a preview of the selected template.
2. In the Name list, do one of the following:
Double-click the name of the template.
Click the Edit Template button at the bottom of the Assets panel.
3. Edit the template in the Document window. Create editable regions in the template as desired
4. Save the edited template by choosing File > Save.
Help files - you've got to love them...