When you open a new workbook in Microsoft Excel, the default file format is the most current Excel file format. This is usually the .xlsx file format. You can change the default file format to another Excel file format, such as .xls or .xlsm.
To set the default file format:
Open Microsoft Excel.
Click the File tab, and then click Options.
In the left pane of the Excel Options dialog box, click Save.
In the Save files in this format list, select the file format that you want to use as the default.
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