Label is getting refresh and storing the old value, how to restrict that from storing the old value?

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ASP.NET

Javascript

The below code returning the total sum of the a particular column in gridview that column consists of textbox control in each and every row..

here im using javascript to calculate the sum.. the process is working fine.. but the problem here is with the label control to which im assigning the result. When i'm assigning the total result to label.. it's showing the exact result for a moment.. fyn.. but after few seconds the label is getting refresh and storing the old value.. how to resolve it..

Note: the label here is under the update panel

here is the label control that which i'm using.

<asp:Label ID="lblTotalAmtText" runat="server" CssClass="mediumblue" Text="0.00" />

What I have tried:

function getsum() {
try {
      var table = document.getElementById('GrdData'); //GrdData is the grid ID
      var sum = 0;
      for (var i = 1; i < table.rows.length; i++)       
      {
        if (!isNaN(table.rows[i].cells[0].innerText)) 
        {
         if (sum == 0) {
         sum =            (parseFloat(table.rows[i].cells[4].firstChild.value)).toString();}
         else {
         sum = (parseFloat(sum) +  parseFloat(table.rows[i].cells[4].firstChild.value)).toString();
         }                       //alert(parseFloat(table.rows[i].cells[4].childNodes[0].value));
        }
      }
     document.getElementById('lbltotalamttext').innerHTML = sum;
    //here is the culprit lbltotalamttext
  }
catch (ex) {
 alert(ex.message)
 }
}