A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I'll take Renzo's word that the iOS and Android versions can't save to CSV. I also tried to save as CSV from Excel Online. It is not an option.
.
On the other hand, if you open the file in Google Spreadsheet, there you can save to CSV.
.
So the process would be to upload the file from your phone to Google drive, open in Google spreadsheet, then SAVE AS to CSV. From there you can download to your phone or MS OneDrive.