JS怎么修改不了GridView里TemplateField模板页的Label控件的值

用JS修改GridView里TemplateField模板页的Label控件的值后又自动刷新变回来了<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript">
        function modify(obj){
            document.getElementById(obj).innerHTML = "100";
         }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <div>
                <asp:Button ID="Button1" runat="server" Text="增加列" OnClick="Button1_Click" />
                <br />
                <br />
                <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
                    onrowdatabound="GridView1_RowDataBound">
                    <Columns>
                        <asp:TemplateField HeaderText="测试列">
                            <ItemTemplate>
                                <asp:Label ID="labName" runat="server" Text='<%# bind("Count")%>'</asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>
                        <asp:TemplateField HeaderText="功能">
                            <ItemTemplate>
                                <asp:Button ID="modify" runat="server" Text="修改" />
                            </ItemTemplate>
                        </asp:TemplateField>
                    </Columns>
                </asp:GridView>
            </div>
        </ContentTemplate>
    </asp:UpdatePanel>
    </form>
</body>
</html>
后台代码//页面加载
protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                this.GridView1.DataSource = null;
                this.GridView1.DataBind();
            }
        }
//在GridView里添加一个空行
protected void Button1_Click(object sender, EventArgs e)
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("Count");
            dt.Rows.Add(dt.NewRow());
            this.GridView1.DataSource = dt;
            this.GridView1.DataBind();
        }protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                Button btn = (Button)e.Row.FindControl("modify");
                Label lab = (Label)e.Row.FindControl("labName");
                btn.Attributes.Add("onclick", "modify('"+lab.ClientID+"');");
            }
        }