What do you mean by 'always have access to it'? Do you mean actually 'always' or just for the duration of the GUI being open.
If you mean always then I would assume xlswrite is the correct approach if your data is from a .xlsx file. I have never used that personally though as I always save data from Matlab to a .mat file using
save( filename, 'variable1', 'variable2', etc)
If you mean you want the data for the duration of the GUI then attach it to the handles struct as e.g
handles.data = ...
guidata( hObject, handles );
Then in any other callback of the GUI you can just access it as