Screen Navigation is controlled either via the Theme settings OR on a topic by topic basis.
THEME SETTINGS
In the 'Settings' screen select Theme > Customise theme settings
In the Topics tab you can specify the default Navigation for the screens in your course
TOPIC SETTINGS
To access the Screen Navigation controls right click on a topic in the Project Structure screen and select 'Topic Settings'
From here you can define the navigation for that specific topicĀ