Flash Dynamic text font change

Upon clicking a button, i want to change the text size of a dynamic text field within a flash document.  Below shows the following code i already have but this does not work.

thanks if you can help!

var format:TextFormat = new TextFormat();
            format.font = "trebuchet";
            format.color = 0xFF0000;
            format.size = 15;
            format.underline = true;
 
            txt_desc.defaultTextFormat = format;

Select allOpen in new window