如何用另一个子元素替换整个子元素？

var parentDiv = document.getElementById("parentDiv");
parentDiv.innerHTML = "<span>新的子元素</span>";

var parentDiv = document.getElementById("parentDiv");
var newChild = document.createElement("span");
newChild.textContent = "新的子元素";
var oldChild = document.getElementById("oldChild");
parentDiv.replaceChild(newChild, oldChild);