If you renamed an Excel file it would still be an Excel file, but Excel may not recognise it as such.
It would be far better to open the file and then go to the File... Save As menu, select the file type as either Text (Tab-delimited) or csv (Comma Separated Values) give it a filename and click Save. You would then be able to open it in any text editor. Note that you can only save one worksheet, so if you had several worksheets you would have to save each one as a separate file.