If you only want to update the values through this function I would advise against the use of an editbox as this might cause you to accidentally change the value yourself. You could just use a textbox and use it's handle to set the string value of this textbox after you have calculated it
so in your callback function add:
set(handles.text1,'String',num2str(a));
set(handles.text2,'String',num2str(b));