You could also change select.options.selectedIndex DOM attribute like this:
function selectOption(index){ _x000D_
document.getElementById("select_id").options.selectedIndex = index;_x000D_
}
_x000D_
<p>_x000D_
<select id="select_id">_x000D_
<option selected>first option</option>_x000D_
<option>second option</option>_x000D_
<option>third option</option>_x000D_
</select>_x000D_
</p>_x000D_
<p>_x000D_
<button onclick="selectOption(0);">Select first option</button>_x000D_
<button onclick="selectOption(1);">Select second option</button>_x000D_
<button onclick="selectOption(2);">Select third option</button>_x000D_
</p>
_x000D_