How to show post in category in last modified date order?

To show post in category in last modified date order, use this code in your category.php or archive.php file.

<?php
// Get the current category
$category = get_queried_object();

// Custom query to get posts in the current category sorted by last modified date
$query_args = array(
    'category_name' => $category->slug,
    'orderby'      => 'modified', // Sort by last modified date
    'order'        => 'DESC',      // Descending order
    'posts_per_page' => -1          // Show all posts
);

$query = new WP_Query($query_args);

// Output the posts
if ($query->have_posts()) :
    while ($query->have_posts()) : $query->the_post();
        ?>
        <h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
        <p>Last updated: <?php the_modified_date(); ?></p>
        <?php
    endwhile;
else :
    echo 'No posts found.';
endif;

// Restore original post data
wp_reset_postdata();
?>