Using ASP.Net AJAX Numeric UpDown Extender inside GridView control

I have text box inside gridview with add button. When I press add button it should increment the text box value. For example I have value 3 in the text box. When I press add it should become now 4 inside gridview row.

    <asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="#3AC0F2" OnRowCommand="GridView1_RowCommand"
            HeaderStyle-ForeColor="White" CssClass="table table-striped table-bordered table-hover"
        AutoGenerateColumns = "false" Width="100%" 
            onrowdeleting="GridView1_RowDeleting">
        <Columns>
            
            <asp:TemplateField HeaderText="Product Code" ItemStyle-Width="150">
                <ItemTemplate>
                    <asp:Label ID="lblCode" runat="server" Text='<%# Eval("ItemCode") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>            
            
            <asp:BoundField DataField="ItemName" HeaderText="Product Name" ItemStyle-Width="200px" />            
            <asp:BoundField DataField="Rate" HeaderText="Price" ItemStyle-CssClass="price" ItemStyle-Width="100px" />
                
                                
                <asp:TemplateField HeaderText="Quantity" ItemStyle-Width="200px">
                    <ItemTemplate>
                        <asp:TextBox ID="txtQuantity" runat="server" class="form-control" Width="100"></asp:TextBox>
                        <asp:HiddenField ID="hdnfldQty" runat="server"  /> 
                        <br/>
                        <asp:Button ID="btnAdd" Text="Add" Class="btn btn-info" runat="server" CommandName="Add"
                        CommandArgument="<%# ((GridViewRow) Container).RowIndex %>" OnClick="btnAdd_Click"  />
                    <asp:Button ID="btnSub" Text="Sub" Class="btn btn-info" runat="server" CommandArgument="btnSub" OnClick="btnSub_Click" />

                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Total" ItemStyle-Width="200px">
                    <ItemTemplate>
                       <asp:Label ID="lblTotal" runat="server" Text="0"></asp:Label>                       
                    </ItemTemplate>
                </asp:TemplateField>

                <asp:CommandField ShowDeleteButton="True" ButtonType="Button"  />
            
        </Columns>
    </asp:GridView>

    protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "Add")
        {
            int index = Convert.ToInt32(e.CommandArgument.ToString());
            GridViewRow gvRow = GridView1.Rows[index]; 
            string qty = (gvRow.FindControl("txtQuantity") as TextBox).Text;
            int ans = Convert.ToInt32(qty);
            int num = 1;
            int result = ans + num;
            Response.Write("Ans:" + result);
        }
    }