<select onchange="selectChange()" id=file name="file" class="d-n">
  <option value=""></option>
  <option value="2">2/option>
    <!-- выделим для наглядности -->
  <option value="3" selected>3</option>
  <option value="4" id="4" class="d-n">4</option>
  <option value="5" id="5">5</option>
</select>
<button id="btn">Reset select</button>

btn.addEventListener("click", (e) => {
  file.selectedIndex = -1;
});