public bool _visible = false;
    public bool IsVisible
    {
        get
        {
            return _visible;
        }
        set
        {
            _visible = value;
        }

    }

<ItemTemplate>
   <asp:Label ID="Label4" runat="server" Visible='<%# !(bool) IsVisible %>'    
         Text='<%# Bind("max") %>'>
   </asp:Label>
   <asp:TextBox ID="TxtMax" Visible='<%#IsVisible %>' Text='<%#Bind("max") %>'  
         runat="server">
</asp:TextBox>
 </ItemTemplate>

protected void Edit_Button_Click(object sender, EventArgs e)
    {
        _Visible = true;
         GridView1.DataBind();

    }



<H2>To Update All Rows. </H2>

protected void Update_Button_Click(object sender, EventArgs e)
{
for (int i=0;i< GridView1.Rows.Count;i++)
     Gridview1.UpdateRow(i, true); 
     _Visible = false;
     Gridview1.DataBind();

}