<html>
<body>
<select name=abc>
<option value=a>a</option>
<option value=b>b</option>
</select>
<input type=button onclick=hoho() value=改变>
<script>
function hoho(){
document.all.abc.value="b"}
</script>
</body>
</html>
<body>
<select name=abc>
<option value=a>a</option>
<option value=b>b</option>
</select>
<input type=button onclick=hoho() value=改变>
<script>
function hoho(){
document.all.abc.value="b"}
</script>
</body>
</html>
<form name="form1">
<select name=abc>
<option value=a>a</option>
<option value=b>b</option>
</select>
<script>
function hoho(){
document.forms["form1"].elements["abc"].value="b";
//或者：
//document.forms["form1"].elements["abc"].selectedIndex=1;
}
</script>