https://stackoverflow.com/questions/44290279/knockoutjs-how-to-reset-select-statement

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.1/knockout-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout.mapping/2.4.1/knockout.mapping.min.js"></script>
<label>Gender:</label> 
<select data-bind="options: gender, value: selectedGender, optionsValue: 'gender', optionsText: 'value', optionsCaption: 'Choose Contact...'"></select>

<br/><br/>
<input type="button" data-bind="click: clear" value="Clear Value" />

JavaScript

var viewModel = function(){
	var self = this;
  
  this.gender = ko.observableArray([{gender: 'F', value: 'female'},{gender: 'M', value: 'male'}]);
  
  this.selectedGender = ko.observable();


  this.clear = function(){
  	self.selectedGender(null);
  }
}

ko.applyBindings(new viewModel());