code: public ApiApp updateApiApp(ApiApp app) throws HelloSignException {
          if (!app.hasClientId()) {
              throw new HelloSignException("Cannot update an ApiApp without a client ID. Create one first!");
          }
          String url = BASE_URI + API_APP_URI + "/" + app.getClientId();
          return new ApiApp(httpClient.withAuth(auth).withPostFields(app.getPostFields()).post(url).asJson());
      }
docs: Updates the API app with the given ApiApp object properties.
  
  @param app ApiApp
  @return ApiApp updated ApiApp
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.