How to Update DataTable ?

<asp:GridView ID="GridView1" runat="server" Width="100%" CssClass="scrtablegrid" GridLines="None" 
                onrowdatabound="GridView1_RowDataBound" ShowFooter="True" 
                AutoGenerateColumns="False" onrowupdating="GridView1_RowUpdating" >
            <AlternatingRowStyle CssClass="scrgridbg2" />
            
            <HeaderStyle CssClass="scrgridheader" Height="30px" HorizontalAlign="Left" Font-Underline="true" ForeColor="White"/>
            <RowStyle CssClass="scrtablegrid" Height="40px" HorizontalAlign="Left" />
            <Columns>
                <asp:BoundField DataField="Rno" HeaderText="#" />
                <asp:TemplateField HeaderText="Item">
                     <ItemTemplate>
                        <asp:Label ID="Label2" runat="server" Text='<%# Bind("Item") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Qty">
                    
                    <ItemTemplate>
                    [<a href="#" class="removeqty">Remove</a>]<br />
		            <asp:TextBox ID="txtQty" runat="server" Text='<%# Bind("Qty") %>'	class="qtyinput" />
                        
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:BoundField HeaderText="Available" />
                <asp:BoundField DataField="Price" HeaderText="Price" DataFormatString="{0:c}" />
                <asp:BoundField HeaderText="Total" DataField="Total" />
            </Columns>
            <FooterStyle CssClass="mitemstotalbg" Height="30px" HorizontalAlign="Left" />
        </asp:GridView>

public void btnGo_Click(object sender, EventArgs e)
    {
        if (Session["CurrentData"] != null)
        {
            DataTable dt = (DataTable)Session["CurrentData"];
            int count = dt.Rows.Count;
            BindGrid(count);
        }
        else
        {
            BindGrid(1);
        }
    
    }

private void BindGrid(int rowcount)
    {
        
        string cs = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(cs))
        {
            string itemCode = txtItemCode.Text;
            string query = "select  ISNULL(ItemCode+' ','')+'<br />'+ISNULL(Name+' ','') as Item,Isnull(left(Price,4),0) as Price  from Y_ProductItem where ItemCode='" + @itemCode + "'";
            SqlCommand cmd = new SqlCommand(query, con);
            cmd.Parameters.AddWithValue("@itemCode", itemCode);
            con.Open();
            SqlDataReader dataReader = cmd.ExecuteReader();
            while (dataReader.Read())
            {
                Session["Item"] = dataReader["Item"];
                Session["Price"] = dataReader["Price"];
            }

            DataTable dt = new DataTable();
            DataRow dr;
            dt.Columns.Add(new System.Data.DataColumn("Rno", typeof(String)));
            dt.Columns.Add(new System.Data.DataColumn("Item", typeof(String)));
            dt.Columns.Add(new System.Data.DataColumn("Qty", typeof(String)));
            dt.Columns.Add(new System.Data.DataColumn("Available", typeof(String)));
            dt.Columns.Add(new System.Data.DataColumn("Price", typeof(String)));
            dt.Columns.Add(new System.Data.DataColumn("Total", typeof(String)));
            GridView1.DataSource = dt;
            GridView1.DataBind();
            if (Session["CurrentData"] != null)
            {
                for (int i = 0; i < rowcount + 1; i++)
                {
                    dt = (DataTable)Session["CurrentData"];
                    
                    if (dt.Rows.Count > 0)
                    {

                        
                      
                        dr = dt.NewRow();
                      

                      

                        dr[0] = dt.Rows[0][0].ToString();
                        dr[1] = dt.Rows[0][1].ToString();
                        dr[2] = dt.Rows[0][2].ToString();
                        dr[3] = dt.Rows[0][3].ToString();
                        dr[4] = dt.Rows[0][4].ToString();
                        dr[5] = dt.Rows[0][5].ToString();


                       
                    }
                }
                dr = dt.NewRow();
                dr[0] = 1;
                dr[1] = Convert.ToString(Session["Item"]);
                 dr[2] = txtQuantity.Text;
                // dr[3] = TextBox4.Text;
                 dr[4] = Convert.ToString(Session["Price"]);
                 float q = Single.Parse(txtQuantity.Text);
                 float d = Single.Parse(Convert.ToString(Session["Price"]));

                 dr[5] = d * q;
                 for (int r = 0; r < rowcount + 1; r++)
                 {

                     dr[0] = r + 1;
                 }
                dt.Rows.Add(dr);

            }
            else
            {
                dr = dt.NewRow();
                dr[0] = 1;
                dr[1] = Convert.ToString(Session["Item"]);
                dr[2] = txtQuantity.Text;
                // dr[3] = TextBox4.Text;
                dr[4] = Convert.ToString(Session["Price"]);
                float q =Single.Parse(txtQuantity.Text);
                float d = Single.Parse(Convert.ToString(Session["Price"]));
               
                dr[5] = d * q;
                dt.Rows.Add(dr);

            }

            // If Session has a data then use the value as the DataSource
            if (Session["CurrentData"] != null)
            {
                GridView1.DataSource = (DataTable)Session["CurrentData"];
                GridView1.DataBind();
            }
            else
            {
                // Bind GridView with the initial data assocaited in the DataTable
                GridView1.DataSource = dt;
                GridView1.DataBind();

            }
            // Store the DataTable in Session to retain the values
            Session["CurrentData"] = dt;

        }
    }

protected void btnUpdate_Click(object sender, EventArgs e)
    {

       // I want to implement here when update...
        
    }