Hoi Taco.

You mean menu items? Or Mega Menu structure ?

If 1st in Menus > Main Menu

If 2nd please check Template Options > Menu

https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/menu-navigation

Paul when I look in template options at basic I see 3 predifined headers there. at layout options the header is not present. where can i change the predefined

In predefined header - you cannot see header on the Layout - but it's used/displayed on site preview. It's that area where you have logo and menu.

But anytime, you can disable predefined header and build your own using Helix documenation guide >> https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/custom-header