The only thing your code is doing is...
... using an interface so the user can select a file
handles.fileName = uigetfile('*.xlsx');
...saving the handles to the button object (which isn't necessary to do)
guidata(hObject, handles);
...reassigning a variable stored in handles.fileName to a new variable named fileName.
fileName = handles.fileName;
The code isn't loading any data. Even if you did load data, you'd have to do something with that data within that function. When the callback function completes, that workspace is no longer available.
