change one fontface to another fontface in c# wpf

Question

I want to change one LTibetan fontface to Arial black fontface in c# WPF but rest Fontfaces are remains same as. I have used below code to achieve the purpose:So anybody kindly give me a help

void ChangeFontFamily(object sender, RoutedEventArgs e)

        {

            FontFamily ffamily = new FontFamily();

            ffamily = btn2.FontFamily;

            ffamily = richTextBox1.FontFamily;

           string str = ffamily.ToString();

           if (str == ("LTibetan"))

            {

                btn2.FontFamily = new FontFamily("Arial Black");

                richTextBox1.FontFamily = new FontFamily("Arial Black");

                btn2.Content = " Arial Black";

                        }