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add_action( 'pre_get_posts', 'my_change_sort_order'); 
function my_change_sort_order($query){
    if(is_archive()):
     //If you wanted it for the archive of a custom post type use: is_post_type_archive( $post_type )
       //Set the order ASC or DESC
       $query->set( 'order', 'ASC' );
       //Set the orderby
       $query->set( 'orderby', 'title' );
    endif;    
};