function listboxSelectDeselect(listID, isSelect) {
  var listbox = document.getElementById(listID);
  for(var count=0; count < listbox.options.length; count++) {
      listbox.options[count].selected = isSelect;
  }
}

Click below buttons to select or deselect all options from select box
<br>
  <select id="lsbox" name="lsbox" size="10" multiple="">
    <option value="1">India</option>
    <option value="2">United States</option>
    <option value="3">China</option>
    <option value="4">Italy</option>
    <option value="5">Germany</option>
    <option value="6">Canada</option>
    <option value="7">France</option>
    <option value="8">United Kingdom</option>
  </select> <br>
<button onclick="listboxSelectDeselect('lsbox', true);">Select All</button>
<button onclick="listboxSelectDeselect('lsbox', false);">Deselect All</button>
<script>
function listboxSelectDeselect(listID, isSelect) {
  var listbox = document.getElementById(listID);
  for(var count=0; count < listbox.options.length; count++) {
      listbox.options[count].selected = isSelect;
  }
}
</script>