A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
If you don't have the desktop software you can't very well open the Excel file with it. The desktop software is not included in the Basic subscription so you only have the Online web apps to work with.
Go to the File tab in Excel Online then use the Export> Download as CSV option. Any text-based program (such as Notepad) will open a CSV file — CSV simply is a specific type of text file. It then can be saved as .txt or several other file formats if you wish.
Note, however, that no calculations will be preserved in the exported file but their results will be displayed as values. Graphic content (such as Charts or Shapes) will be excluded as well.