Microsoft Excel is a spreadsheet application that is part of the Microsoft Office product suite. The extension .xls denotes that the file is a Microsoft Excel file. With the launch of Office 2007, Microsoft changed the default file extension to .xlsx. This new file format is part of Microsoft's Open XML format, which results in a smaller file size and better data integration with external sources. If you have a file with the extension .xls, you can easily convert it to the newer .xlsx file format.
Open the file that you want to convert in Excel.
Click the round "Office" button in the top left corner of the screen to open the file menu.
Click "Save As."
Select "Excel Workbook" from the menu. Select the location where you want to save your file, give it a name and click "Save." Your file will now be saved with the file extension .xlsx instead of .xls.
Saving a file in a different format can sometimes affect the formatting. After you have saved it, look over it carefully to make sure no data has been lost.