SOURCE

function selectOrange()
  {
  document.getElementById("orange").selected=true;
  }

<form>
   请选择您喜欢的水果： 
  <select> <option id="apple">苹果</option> <option id="orange">桔子</option> <option id="pineapple" selected>菠萝</option> <option id="banana">香蕉</option> </select> 
  <br>
  <br> 
  <input type="button" onclick="selectOrange()" value="选择桔子"> 
 </form>