How can I replace dashboard.css and dashboard.min.css (not add new css)

Because wp_dequeue_style should run after wp_enqueue_style ran. So you need to call it with a lower priority so your hook can be applied after it has been enqueued.

try this

add_action( 'wp_enqueue_scripts', 'my_dequeue_scripts', 99 );
function my_dequeue_scripts(){

  wp_dequeue_style( 'dashboard' );

}