<select id="SetSelect">
   <option value= 1 >1st</option>
   <option value= 2 >2nd</option>
   <option value= 3  selected>3rd</option>
   <option value= 4 >4th</option>
</select>
<input type="text" id="InputTextValueIndex">
<input type="button" value="Set by value" onclick="document.getElementById('SetSelect').value=document.getElementById('InputTextValueIndex').value">
<input type="button" value="Set by selectedIndex" onclick="document.getElementById('SetSelect').selectedIndex=document.getElementById('InputTextValueIndex').value">