A file with the file extension of ".BAK" is a backup file. The BAK file extension is a generic file extension to indicate that the file is a backup file. However, if you know the BAK file was created in Microsoft Excel, you can rename the file extension to an Excel-compatible file type, such as XLS or XLSX, to be opened in Excel. If the BAK is an Excel file, renaming the file extension will convert the file to the Excel format.
Right-click on the Windows "Start" button, and open the Windows Explorer application.
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Click the "Tools" option, then click "Folder Options."
Click the "View" tab, then uncheck the "Hide Extensions for Known File Types" checkbox. Click the "OK" button.
Navigate to the BAK file and right-click on the file.
Click "Rename" in the context menu. The file name, along with the file extension, is highlighted.
Move the cursor to highlight only the ".BAK" file extension, and type either ".XLS" or ".XLSX," depending on the version of Excel was used to create the file.
Double-click on the renamed file. If the file is an Excel file, the data will open in an Excel spreadsheet.