A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Check your default SaveAs setting:
[File] Tab, "Options" choice.
In the Excel Options window, select the "Save" item and look to see what the very first setting on the page is, next to Save files in this format:. If it happens to be Excel 97-2003 Workbook (*.xls) then change it to something else that you prefer from the list of available formats.
If you make a change, I recommend then closing Excel and reopening it before depending on the setting to take effect for new workbooks.
For existing workbooks, if you open a .xls file, it is probably going to just save it as a .xls type unless you take action to File --> SaveAs to force it to a different type.