The Main Menu is the first screen of the Roadbook Designer. All the other screens are opened from here. To come back to it from any screen, tap the Designer icon at the top left.
The first time you open the app, the Designer asks for the permissions it needs:
Tap Proceed and accept the permissions.
On the Navigation screen the phone's Back button does nothing, so that you don't lose your work with an accidental tap. Use the Designer icon to return to the Main Menu.