<form>
      <select id="action">
         <option value="a">Add</option>
         <option value="b">Delete</option>
      </select>
 </form>

<a href="#" onClick="document.getElementById('action').value='a'">Add</a> 
<a href="#" onClick="document.getElementById('action').value='b'">Delete</a> 

或者U也可以调用此Java Script函数来执行此操作=

OR U can also call this Java Script function for doing this =

function changeval()
{

if(document.getElementById('action').value =='b')
document.getElementById('action').value='a'
else
document.getElementById('action').value='b'

}

<a href="#" onClick="changeval()">Add</a> 
<a href="#" onClick="changeval()">Delete</a>