Change Post Order in WordPress Archives

The simplest way to change the post order on the page that uses the archive.php file is to use the pre_get_posts hook. In this case you should make sure the query is the one you want to apply the change to.

Using the is_archive or is_post_type_archive condition should work in this case. Here is an example:

function sv_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;
}
add_action( 'pre_get_posts', 'sv_change_sort_order');

You may want to learn more about the WordPress template hierarchy if you are not sure what this means.

Alternatively, if you prefer a graphical way to manage the order, you can use the dedicated plugin – reordering custom post types in WordPress which offers a drag-and-drop interface directly in the dashboard.