Unable to set focus in TextBox inside ASP.Net UpdatePanel

Then another way is JavaScript

 <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:TextBox ID="TextBox1" runat="server" OnTextChanged="TextBox1_Changed" AutoPostBack = "true"></asp:TextBox>
            <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        </ContentTemplate>
    </asp:UpdatePanel>

  protected void TextBox1_Changed(object sender, EventArgs e)
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "f", "document.getElementById('" + TextBox2.ClientID + "').focus();", true);
    }