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);
}
}