<header>
  <div> I'm a text ! </div>
  <button> Toggle text </button>
</header>

body, html {
  margin: 0;
}

header {
  width: 100%;
  height: 200px;
  background: royalblue;
  text-align: center;
  color: white;
  font-size: 33px;
}

div {
  margin: 20px;
}

var el = document.querySelector('div');
document.querySelector('button').addEventListener('click', function() { 
  el.style.display = el.style.display === 'block' ? 'none' : 'block';
});