This is not a supported modification in powerstore and wil require some hand coding to accomplish.
you would need to add the service select list to the checkout page:
<select name="UPS_Service" id="UPS_Service">
<option value="01">Next Day Air</option>
<option value="02">2nd Day Air</option>
<option value="03" selected>Ground</option>
<option value="07">Worldwide Express</option>
<option value="08">Worldwide Expedited</option>
<option value="11">Standard</option>
<option value="12">3 Day Select</option>
<option value="13">Next Day Air Saver</option>
<option value="14">Next Day Air Early AM</option>
<option value="54">Worldwide Express Plus</option>
<option value="59">2nd Day Air AM</option>
<option value="65">Express Saver</option>
</select>
and add a hidden form element to capture the selected service on the confirm page:
<input name="UPS_Service" type="text" id="UPS_Service" value="<?php echo((isset($_POST["UPS_Service"]))?$_POST["UPS_Service"]:"") ?>" />
then edit the ups shipping code to use the sellected service:
$Shipping_Parameters["CarrierCode"] = "".($_POST["UPS_Service"]) ."";
the issue you will run into is that not all services are valid in some shipping scenarios.