Try this one friend.......
------------
Aspx Page
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<asp:label id="_lbl" runat="server" text="Check" xmlns:asp="#unknown">
<asp:button id="_btn" runat="server" text="Hit..." onclick="_btn_Click" xmlns:asp="#unknown">
------------
Code Behind .cs file which find control
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protected void _btn_Click(object sender, EventArgs e)
{
Label lbl=(Label)_d.FindControl("_lbl");
lbl.Text = "Hiiiiiiiiiiiiiiiii";
}
Enjoy solution :):):)........