code: private function _refreshToken()
      {
          $requestUrl = $this->config->getHost() . "/oauth2/token";
          $postData = "grant_type=refresh_token&refresh_token=" . $this->config->getRefreshToken();
          $response = $this->client->send(new Request('POST', $requestUrl, [], $postData));
          $result = json_decode($response->getBody()->getContents(), true);
          $this->config->setAccessToken($result["access_token"]);
          $this->config->setRefreshToken($result["refresh_token"]);
      }
docs: /*
  Refresh token