How can I convert excel extension to .csv? Thanks

It's one of the FILE TYPE options available in the SaveAs window.

1) When you're data is in the state you wish to save, choose SaveAs instead of Save (I usually just press F12)
2) In the Save as type: line select CSV (comma delimited) (*.csv)
3) Enter your File name:
4) Click OK
5) Excel will warn you that the CSV format will not retain any formatting, click OK