By default, Wordpress sorts blog posts by creation date. However, if you update your blog posts from time to time, you may want to sort them by modification date rather than creation date.

To achieve this, use this snippet:

function order_posts_by_mod_date($orderby) {
  if  (is_home() || is_archive() || is_feed()) {
    $orderby = "post_modified_gmt DESC";
  }

  return $orderby;
}

add_filter('posts_orderby', 'order_posts_by_mod_date', 999);

In your theme, just dump this snippet into functions.php. (You may need to create this file in your theme’s directory.)