How do you display random posts in Landing Page > Feature Boxes?

The theme is designed to display the most recent posts (sorted descending by publish date).

If you need some other order criteria (including random) you’d need to use the theme’s fluida_boxes_query_args filter to customize the query:

add_filter( 'fluida_boxes_query_args', 'child_custom_boxes_order', 12, 2 );
function child_custom_boxes_order( $args=array() ) {
    $custom_order = array(
        'orderby' => 'rand', 
        //'order' => 'DESC',
    );
    return array_merge( $args, $custom_order );
} // child_custom_boxes_order()

You use that code in a child theme.