1) Be sure to pass the "handles" variable to your subfunctions, so they can "communicate" with your GUI as properties (e.g. string, font size, font color, etc) get updated
Typically Callbacks for buttons, etc, look like:
function pushbutton1_Callback(hObject, eventdata, handles)
2) Assuming your static text box has the tag "update_text"
set(handles.update_text,'String',"Insert updated string here"); %sets new property for static_text
guidata(hObject,handles); %updates the figure with new "handles" information
drawnow;
