If it is a static .html page, you need to enable the script hidden elements.
In the Dreamweaver preferences, select the Invisible Elements section, and check Scripts. Next to the menu, will be displayed a shield icon for the scripts, select the scripts shield icon to trigger the edit option in the property inspector.
If it is a dynamic .php page, selecting the menu in design view should show the edit option in the property inspector.
To edit the menu for the site pages, you need to edit the template.
to edit the menu for the home page, you can edit the index.php page.