1. open App.js 

You should see the pre-included code

import React from 'react';
import logo from './logo.svg';
import './App.css';

function App() {
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <a
          className="App-link"
          href="https://reactjs.org"
          target="_blank"
          rel="noopener noreferrer"
        >
          Learn React
        </a>
      </header>
    </div>
  );
}

export default App;

2. replace everything with just this 

import React from 'react';

const App = () => {
  return <h1>Hello World!</h1>
}

export default App;

3. Save the file 

4. Open the terminal in this directory and run 

npm start

5. Now open your web browser and go to this link http://localhost:3000/. You should see something like this

Hurray 🥳 You have edited your own ReactJS application. Pat your back and get ready for even more action. 

Note -

function App () {
}

and 

const App = () => {
}

Are both valid ways of defining a function in JavaScript ES6. For this series we will be using the second method also called as "fat arrow" method as it provides some additional benefits over simple functions.