Hi you can add following code on your dropdownlist control selectedindexchanged event
If you have lable on your page and you want to change its font style dynamically use following code
<asp:Label ID="Label1" runat="server" Text="Label">Hi this is text</asp:Label>
if (Label1.Style[HtmlTextWriterStyle.FontFamily] == null)
Label1.Style.Add(HtmlTextWriterStyle.FontFamily, "TIMES NEW ROMAN");
else
Label1.Style[HtmlTextWriterStyle.FontFamily] = "Calibri";