Grid view last cloumn cell edite

<asp:GridView ID="GRVItem" DataKeyNames="ItemCode,ItemName" runat="server" Font-Names="Calibri"
                        Font-Size="11pt" AlternatingRowStyle-BackColor="#C2D69B" AllowPaging="True" PageSize="5"
                        ShowHeader="true" CssClass="mGrid" PagerStyle-CssClass="pgr" AlternatingRowStyle-CssClass="alt"
                        Width="1000px" padding-top=" 70px;" AutoGenerateColumns="false" >
                        <HeaderStyle BorderColor="Gray" BorderStyle="Solid" Font-Bold="true" BackColor="#76C3FD" />
                        <Columns>
                            <asp:TemplateField>
                                <HeaderTemplate>
                                    <asp:CheckBox ID="chkAll" runat="server" AutoPostBack="true" OnCheckedChanged="CheckBox_CheckChanged" />
                                </HeaderTemplate>
                                <ItemTemplate>
                                    <asp:CheckBox ID="chk" runat="server" AutoPostBack="true" OnCheckedChanged="CheckBox_CheckChanged" />
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:BoundField HeaderText="Item Code" DataField="ItemCode"  ReadOnly="true"/>
                            <asp:BoundField HeaderText="Item Name" DataField="ItemName" ReadOnly="true" />
                            <asp:BoundField HeaderText="In Stack" DataField="OnHand" ReadOnly="true" />
                            <asp:TemplateField HeaderText="Quantity">
                                <EditItemTemplate>
                                    <asp:TextBox ID="txtQuantity" runat="server" />
                                </EditItemTemplate>
                            </asp:TemplateField>
                        </Columns>
                    </asp:GridView>