update_post_meta not working?

I’m not quite sure about what you want to do…
Maybe it’s this:

add_action('save_post', 'update_thumb');

function update_thumb($post_id) {

    //verify post is not a revision
    if (! wp_is_post_revision($post_id)) {

        $new_value="testxxx";
        update_post_meta($post_id, 'thumb', $new_value); 

    }

}