how to: select onchange alter another field on same page

[jasonc]

I have a multiple fields on my page and wish to have a reusable javascript to update the previous box with the selction made.

 

how would this be done

 

<form name="form1" method="post" action="">
  <p>
    <input type="textone" name="textfieldone" id="textfieldone">
  </p>
  <p> 
    <select name="selectone" onChange="UpdatePreviousField()">
      <option value="1">1</option>
      <option value="2">2</option>
      <option value="3">3</option>
    </select>
  </p>
  <p>  </p>
  <p>  </p>
  <p>  </p>
  <p>  </p>
  <p> 
    <input type="texttwo" name="textfieldtwo" id="textfieldtwo">
  </p>
  <p> 
    <select name="selecttwo" onChange="UpdatePreviousField()">
      <option value="1">1</option>
      <option value="2">2</option>
      <option value="3">3</option>
    </select>
  </p>
  <p>  </p>
</form>