Hi there,
you will need to do the following:
1. In Customizer > Layout > Primary Navigation set the Location to After Header
2. add the following PHP Snippet to your site:
add_action( 'after_setup_theme','craig_move_navigation' );
function craig_move_navigation() {
remove_action( 'generate_after_header', 'generate_add_navigation_after_header', 5 );
add_action( 'generate_after_header', 'generate_add_navigation_after_header', 15 );
}
This article explains how to add PHP:
https://docs.generatepress.com/article/adding-php/
Please pass this response on to your developer.