into the file category.php use for exemple

before loop

  and insert the function below

into the file category.php use for example

sort_query_posts_by('title', 'desc');

or use ID, date etc. (see the function below) before loop and insert the function below

if (! function_exists('sort_query_posts_by'))
{
    function sort_query_posts_by($order_by, $order = 'asc')
    {
        global $wp_query;
        $order_by = strtolower($order_by);
        $order    = strtolower($order);

        if ($order_by == 'rand') {
            shuffle($wp_query->posts);
            return;
        }

        if ($order_by == 'none') {
            $order_by = 'id';
            $order = 'asc';
        }

        $props = array(
            'author'        => 'return sqp_compare_by_number($o1->post_author, $o2->post_author, '.$order.');',
            'comment_count' => 'return sqp_compare_by_number($o1->comment_count, $o2->comment_count, '.$order.');',
            'date'          => 'return sqp_compare_by_number(strtotime($o1->post_date), strtotime($o2->post_date), '.$order.');',
            'id'            => 'return sqp_compare_by_number($o1->ID, $o2->ID, '.$order.');',
            'menu_order'    => 'return sqp_compare_by_number($o1->menu_order, $o2->menu_order, '.$order.');',
            'modified'      => 'return sqp_compare_by_number(strtotime($o1->post_modified), strtotime($o2->post_modified), '.$order.');',
            'parent'        => 'return sqp_compare_by_number($o1->post_parent, $o2->post_parent, '.$order.');',
            'title'         => 'return sqp_compare_by_string($o1->post_title, $o2->post_title, '.$order.');'
        );

        usort($wp_query->posts, create_function('$o1, $o2', $props[$order_by]));
    }

    function sqp_compare_by_number($n1, $n2, $order)
    {
        $n1 = (int) $n1;
        $n2 = (int) $n2;
        $v  = $n1 > $n2 ? 1 : ($n1 < $n2 ? -1 : 0);
        return ($order == 'desc') ? $v * -1 : $v;
    }

    function sqp_compare_by_string($s1, $s2, $order)
    {
        $v = strnatcasecmp($s1, $s2);
        return ($order == 'desc') ? $v * -1 : $v;
    }
}