<select id="PaymentMode" class="selectpicker" name="PaymentMode" tabindex="-98">
<option value="">-- Please Select --</option>
<option value="123" selected="selected">Cash</option>
<option selected="selected" value="456">Transaction Transfer</option>
</select>

onchange="changePaymentMode(event,this)"

function changePaymentMode(evt, obj){

          var isMoreThan2000 = 2000;
          var requiredAmount = $("input[id$='txtRquiredAmount']");

          console.log(requiredAmount.val());

          if(parseInt(requiredAmount.val()) < isMoreThan2000)
          {
              console.log("true");
              var s = $('[id$=dplPaymentMode] option:nth-child(2)').attr("selected", "selected").text();  //verification purpose : gets the text
              console.log(s)
              //$('[id$=dplPaymentMode] option:nth-child(2)').attr("selected", "selected").text(); // <-- doesn't select
          }

        }