var range = document.createRange();
var p1 = document.getElementById("p1");
var helloNode = p1.firstChild.firstChild;
range.setStart(helloNode, 1);
range.setEnd(helloNode, 4);
// 새 노드 생성
var newNode = document.createElement("span");
newNode.style.color = "orange";
range.surroundContents(newNode);
<p id="p1"><b>Hello</b> world!</p>