insertBefore

node1

node2

function myFunction() {
  // Creates new list item
  let newItem = document.createElement("li");
  newItem.innerHTML = "Water";
  
  // Puts list item into myList before "tea" item
  let list = document.getElementById("myList");
  list.insertBefore(newItem, document.getElementById("tea"));
}

<ul id="myList">
  <li>Coffee</li>
  <li id="tea">Tea</li>
  <li>Cocacola</li>
</ul>

<p>Click the button to insert an item to the list.</p>

<button onclick="myFunction()">Try it</button>