Set “editor” role to existing user

Usermeta key that stores the user’s role has a prefix in the name, the one set in the wp-config.php file. So meta_key will have the form {prefix}_capabilities and your query should look like

global $wpdb;
$wpdb->query(
        $wpdb->prepare(
          "UPDATE {$wpdb->usermeta} SET meta_value="a:1:{s:6:\"editor\";b:1;}" WHERE user_id = %d AND meta_key = '{$wpdb->prefix}capabilities'",
          $user_id ) 
);

You do not need to write SQL query. You can achieve the same by fetching the WP_User object with get_userdata function and set new role with the method set_role (or remove_role and add_role to change only one of the roles).

$my_user_id = 12345;   // user ID for changing role
$my_user = get_userdata($my_user_id);
if ( $my_user instanceof WP_User )
    $my_user->set_role('editor');