You should use
$('#dropdownid').val('selectedvalue');
Here’s an example:
$('#dropdownid').val('selectedvalue');
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id='dropdownid'>
<option value="">- Please choose -</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="selectedvalue">There we go!</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>