Convert BAK to XLSX

It is only possible to recover / convert .bak files as excel spreadsheets (.xlsx) if these backups actually contain some kind of table, spreadsheet or database in first place. In most cases just the file extension is renamed and nothing else is done with the file, so just renaming the extension back should be enough. If that  doesn't work, it is likely that the recovery can only be done with the software that made the backup in first place, it is unlikely some universal converter for bak to xlsx conversion exists.