Convert BAK to DOCX

It is only possible to convert .bak files as Word documents (.docx) if these backups actually contain some kind of document in first place. Typically, programs simply rename the extension from original to bak, so no conversion is needed, just rename the file back.

If this doesn't work, then you probably have no way how to reliably convert the data, because it is unlikely that your .bak file is actually MS Word document.