In Excel, workbooks are saved as Excel Workbooks (.xlsx) by default. You can change the default file format to any other file format that is supported in Office Excel 2007. For example, if you frequently create macros in a workbook, you may want to automatically save a new workbook as an Excel Macro-Enabled Workbook (.xlsm). Or, if you work with others who use earlier versions of Microsoft Excel, you may want to save a new workbook as an Excel 97-2003 Workbook (.xls).
In Excel 2007: Click the Microsoft Office Button , and then click Excel Options.
In all other Excel versions: click File > Options.
In the Save category, under Save Workbooks, in the Save files in this format list, click the file format that you want to use.
Tip: For more information, see File formats that are supported in Excel.
After you specify the default file format, all new workbooks are saved in the selected file format unless you specify a different format in the Save As dialog box when you save a workbook.
Some features and formatting might not be available if you save the workbook in the file format of an earlier version of Microsoft Excel or another spreadsheet program.
For more information, see Excel formatting and features that are not transferred to other file formats.