<button>Switch it</button>
<p>Awesome</p>

<script>

var btn = document.querySelector('button');
var txt = document.querySelector('p');

btn.addEventListener('click', updateBtn);

function updateBtn() {
  if (txt.textContent === 'Awesome') {
    txt.textContent = 'It\'s Not Awesome';
  } else {
    txt.textContent = 'Awesome';
  }
}

</script>