<select>
<option value="1" selected="selected">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<select>
<option value="1" selected="selected">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<button>Reset</button>
const $select = document.querySelector('select');
document.querySelector('button').addEventListener('click', () => {
$select.value = $select.querySelector('option[selected]').value;
});