document.querySelector("#combobox").options[2].selected = true;
<body>
<select name="" id="combobox">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<button onclick="Change()">изменить</button>
</body>
<script>
async function Change(){
let s = document.getElementById("combobox")
s.value = 2
console.log(s.value);
}
</script>