Getting the model to use the latest excel spreadsheet.

I have a question about updating daily data from excel to powerbi. I have 2 sheets that were used to build the model call them oct13 and oct13all. Now I want to bring in the latest days data which are on sheets Oct14 and Oct14all. These load fine in power bi but how do i ask powerbi to replace the Oct13 sheet with the latest sheet called Oct14? I am not saving them as tables, just simple spreadsheets,   

Thanks George