Display Archive pages by Modified Date in WordPress

functions.php

add_action( 'pre_get_posts', 'sk_order_posts_by_modified' );
/**
 * Arrange Posts by last modified with the most recently updated ones at the top
 * 
 * @author Sridhar Katakam
 * @author Bill Erickson <- Boss
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data
 *
 */
function sk_order_posts_by_modified( $query ) {
	
	if( $query->is_main_query() && !is_admin() && is_archive() ) {
		$query->set( 'orderby', 'modified' );
	}

}