Hi @howie475,
Go to Appearance → Customize → Additional CSS (the menu labels might be different in your language) and add the code below:
div.navigation { position: fixed; left: 0; top: 0; right: 0; }
You might want to wrap it in a media query, to prevent issues with the mobile menu. You might also need to adjust the z-index value of the navigation, in case elements further down the page are displayed on top of it.
— Anders