The first problem you need to solve is how to download the Excel file from your Application Under Test (AUT) to your local PC. Unless you get the file in your hand, you can do nothing on it.
How can you download the file from you AUT? — it depends on how the web page is designed, there could be a lot of variations. Please try to develop a test script to download the file for yourself. If you encounter any problem, of course you can ask questions providing enough description. Please have a look at