Screen Navigation is controlled either via the Theme settings OR in the Topic Settings from the course structure page.
In the Course Structure page, select the 'Course Settings' tab, screen select Theme > Customise theme settings.
In the Topics tab you can specify the default Navigation for the screens in your course
To access the Screen Navigation controls, right click on a topic in the Course Structure screen and select 'Topic Settings'
From here you can define the navigation for that specific topic