1) the old data on that xls(x) file is deleted or simply they add more data to each already existing row? If they simply add new data you just have to load everything in each dashboard (but I assume this is not the case cause it seems very simple).
2) Or each month data is a new xls(x) file with different name?
If the metadata doesn't change you just need to reload your app every month, right after your excel source has been updated. If you have access to the QMC you can schedule a reload task every month (i.e. every 5th at 9 am).