The XLSX file format is used by Microsoft Excel files created in versions 2007 and 2010. Previous versions of the program (97 through 2003) used the XLS extension. Due to the expanded use of the newer versions, you may see more XLSX files. If you are running an older version of Excel, you may have difficulty opening files with the new extension. However, you can open XLSX files with an older version of Excel by following a few steps.
Download the Microsoft Office Compatibility Pack to your computer.
Launch the older version of Excel.
Click the "File" menu and select "Open." Use the "Look in" menu to find the folder in which the XLSX file is saved. Highlight the XLSX file name and click "Open."