To convert a .xlsx or .xls file into a .csv file, Microsoft Excel is the perfect tool.

First, load your .xlsx or .xls file with Excel. Then, use the File -> Save As menu action.

File -> Save As
File -> Save As

Choose where you want to save your CSV file and select CSV UTF-8 (Comma delimited) (.csv) in the File format dropdown.

Select CSV in file formats
Select CSV in file formats

Finally, click the Save button.