You can't change these predefined text boxes in the "Menu Designer" but you can in the "Author" window. Double click in the text box and type away. If you delete all the text in the box you will get a "Double click here to edit" place holder. When you go to "Preview" you will see that this text is not displayed.
You can set font, style, size.... so on for these text boxes in the "Menu Designer". You can also select the text box, say "Item 1" and then untick "apply font face" in the character types and you will get an empty text box as well.
Jeff