wps to docx conversion is essentially attempt to convert old MS Works documents to MS Word format, something that is not that easy nowadays, especially if you do not have access to Works or some older version of MS Word.
Some free alternatives like LibreOffice or OpenOffice.org Writer did support Works formats, at least in older versions so present a viable choice how to convert wps to docx. You can also find plugins for Word, like Microsoft Works 6-9 File Converter that add support for .wks files to Word. Some online conversion services like ZAMZAR should also be capable of help, albeit some errors with formatting might occur.
The last option would be to find installation discs or archives of Works and use that to export the documents to RTF format and import that to modern Word.Open WPS file Open DOCX file
Word processor and text editor from OpenOffice.org suite
One of the many office suite software products
Full-featured word processing and desktop publishing tool from LibreOffice suite
One of the most popular word processor program from Microsoft Office suite
Program used to open Works documents in Microsoft Word