BS4

HTML

<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">


<div id="coupon-options">
<select name="asd" class="form-control" title="" id="gfhfg">
  <option value="1" selected="">Unknown</option>

  <option value="2">Yes</option>

  <option value="3">No</option>

</select>
</div>

JavaScript

$(document).ready(function() {
var els = $('#coupon-options select').find('option[text="Unknown"]');
$.each(els, function(i,v){
$(this).text('sd')
})
})