A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Hi
It all depends on how you downloaded the files. For example, you can't save changes to a file that is attached to an email message. That's the #1 reason, so perhaps this applies to you, as well.
To save the file you must use the Menu Bar and choose File > Save As and choose a different location for the modified workbook.
There are other possibilities such as you are using an unlicensed product, but you didn't say what version of Excel you have or what license type you're using.