you have put your content place holder outside of your form and then close it before you have closed your form
try using this code
i copied your code and tried to run it but there were issues cos it was very neat
<form>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
<asp:Menu ID="Menu2" runat="server" Orientation="Horizontal">
<Items>
<asp:MenuItem NavigateUrl="~/Default.aspx" Text="HOME" Value="HOME"></asp:MenuItem>
<asp:MenuItem NavigateUrl="~/About.aspx" Text="About" Value="About">
</asp:MenuItem>
<asp:MenuItem NavigateUrl="~/Contact.aspx" Text="CONTACT" Value="CONTACT"></asp:MenuItem>
<asp:MenuItem NavigateUrl="~/Change.aspx" Text="CHANGE" Value="CHANGE"></asp:MenuItem> </Items>
</asp:Menu>
</asp:ContentPlaceHolder>
</form>