Untitled

<html>
    <select id="sel" onchange='myFunction()'>
        <option value="car">1</option>
        <option value="bike">2</option>
        <option value="cycle">3</option>
    </select>

    <button onclick='selectChange()'>Click me</button>

    <p id="demo"></p>

    <script>
    function myFunction() {
        document.getElementById("demo").innerHTML = "Hello World";
    }

    function selectChange(){
        document.getElementById('sel').value = "bike";
    }
    </script>
</html>