FВыберите Очистить выделение

$(function() {
  $('.my-select-box').fSelect();

  $('#ClearSelect').click(function() {
    $('#fselectMulti option:selected').removeAttr('selected');

    $('.my-select-box').prev(".fs-dropdown").find(".fs-options .fs-option").each(function() {
      $(this).removeClass('selected', false);
    });

    console.info($('#fselectMulti').val());
    $('.fs-label').html('Select some options');
  })
  $('#CheckSelect').click(function(){
    console.info($('#fselectMulti').val());
  })
});

<script src = "https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link href = "https://facetwp.com/wp-content/plugins/facetwp/assets/vendor/fSelect/fSelect.css" rel = "stylesheet" />
<script src = "https://facetwp.com/wp-content/plugins/facetwp/assets/vendor/fSelect/fSelect.js"></script>
<select id = "fselectMulti" class = "my-select-box" multiple = "multiple">
  <optgroup label = "Northeast">
    <option selected value = "me">Maine</option>
    <option selected value = "ny">New York</option>
    <option value = "nj">New Jersey</option>
    <option value = "vt">Vermont</option>
  </optgroup>
  <optgroup label = "Southwest">
    <option value = "az">Arizona</option>
    <option selected value = "nm">New Mexico</option>
    <option value = "ca">California</option>
    <option value = "nv">Nevada</option>
  </optgroup>
</select>

<button id = "ClearSelect">Clear</button>
<button id = "CheckSelect">Check Array</button>