Change value of Label inside ASP.Net AJAX ModalPopupExtender Modal Popup using JavaScript and jQuery

Here I have created sample that full-fill your requirement.

HTML

<div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
    <script type="text/javascript">
        $(function () {
            $('[id*=lblRechargeDate]').html($('[id*=hfRecharge]').val())
        });

        function SetLabel() {
            $('[id*=divRecharge]').show();
            $('[id*=hfRecharge]').val($('[id*=txtRechargeDate]').val());
        }
    </script>
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />
    <!-- ModalPopupExtender -->
    <cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
        CancelControlID="btnClose" BackgroundCssClass="modalBackground">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">
        This is an ASP.Net AJAX ModalPopupExtender Example WITH Calendar Extender
        <br />
        <asp:TextBox ID="txtRechargeDate" runat="server" CssClass="textbox"></asp:TextBox>
        <cc1:CalendarExtender ID="txtrechargedate_CalendarExtender" runat="server" Enabled="True"
            TargetControlID="txtRechargeDate" PopupPosition="BottomLeft">
        </cc1:CalendarExtender>
        <asp:Button ID="btnClose" runat="server" Text="Close" />
        &nbsp; &nbsp;
        <asp:Button ID="btnOk" Text="Ok" runat="server" OnClick="Show" OnClientClick="SetLabel()" />
    </asp:Panel>
    <br />
    <br />
    <asp:HiddenField ID="hfRecharge" runat="server" />
    <div id="divRecharge" runat="server">
        RechargeDate :
        <asp:Label ID="lblRechargeDate" runat="server" /></div>
</div>

I hope this will help you out.