How to use the aws-api-gateway-client.default function in aws-api-gateway-client

#!/usr/bin/env node

var packageJson = require("./package.json");

var AWS = require("aws-sdk");
var AWSCognito = require("amazon-cognito-identity-js");
var apigClientFactory = require("aws-api-gateway-client").default;
var WindowMock = require("window-mock").default;

global.window = { localStorage: new WindowMock().localStorage };
global.navigator = function() {
  return null;
};

var argv = require("yargs")
  .option("username", {
    describe: "Username of the user",
    demandOption: true,
    string: true
  })
  .option("password", {
    describe: "Password of the user",
    demandOption: true

/*jshint esversion: 6 */
/*jshint node: true */

"use strict";

var fs = require('fs');
var os = require('os');
var aws = require('aws-sdk');
var pty = require('node-pty');
var apiClientFactory = require('aws-api-gateway-client').default;

var buffer = "";
var retrigger = "";
var ptyProcess = {};
var credentialsReady = 0;

var region = fs.readFileSync('/root/region', 'ascii').trim();
var manifest = JSON.parse(fs.readFileSync('/root/manifest.json'));
var keyspace = parseInt(fs.readFileSync('/root/keyspace'));
var apigateway = fs.readFileSync('/root/apigateway', 'ascii').trim();
var manifestpath = fs.readFileSync('/root/manifestpath', 'ascii').trim();
var instance_id = fs.readFileSync('/root/instance_id', 'ascii').trim();
var instance_count = parseInt(fs.readFileSync('/root/instance_count'));
var instance_number = parseInt(fs.readFileSync('/root/instance_number'));

var apiClient = null;