Hi there,
Try this snippet:
add_action( 'after_setup_theme', 'lh_move_secondary_nav' );
function lh_move_secondary_nav() {
remove_action( 'generate_before_header', 'generate_add_secondary_navigation_before_header', 7 );
add_action( 'generate_after_footer_widgets', 'generate_add_secondary_navigation_before_header', 7 );
}
Adding PHP: https://docs.generatepress.com/article/adding-php/