<html>
<select onchange='document.getElementById("myButton1").value="Close Curtain";' onfocus="this.selectedIndex = -1;">
<option>A</option>
<option>B</option>
<option>C</option>
</select>
<input onclick="change()" type="button" value="Open Curtain" id="myButton1"></input>
</html>var map = {
'opt1' : 'Текст 1',
'opt2' : 'Текст 2'
......
}
var sel = document.getElementById( 'my-select' );
var btn = document.getElementById( 'my-button' );
btn.value = map[ sel.value ];