<div id="app">
  <div style="display:flex;">
    <button>Test</button>
      old
  </div>
</div>

<script>
  function go() {
    // Get text node with "old" in it.
    const node = document.getElementById('app').firstElementChild.lastChild;
    // Change to "new".
    node.textContent = 'new';
  }
</script>