How to use WP_Query with have_posts()?

How to use WP_Query with have_posts()?

global $wp_query;

$original_query = $wp_query;
$wp_query = null;
$wp_query = new WP_Query( $args );
if ( have_posts() ) :
    while ( have_posts() ) : the_post();
        the_title();
        the_excerpt();
    endwhile;
else:
    echo 'no posts found';
endif;
$wp_query = null;
$wp_query = $original_query;
wp_reset_postdata();

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