With Microsoft Publisher, users can export their own document files (.pub) to any of the available MS Word formats (.doc, .docx). Just open up the file Publisher and use the File ► Save as.. menu to save your document as .docx.
If you do not have access to Publisher, you will have to rely on some of the 3rd party solutions like OpenOffice or similar. However, it is always best to use the original software because of compatibility reasons and using 3rd party all for pub to docx conversion may result of some data being lost.Open PUB file Open DOCX file
Word processor and text editor from OpenOffice.org suite
Full-featured word processing and desktop publishing tool from LibreOffice suite
A desktop publishing application included in Microsoft Office