A family of Microsoft word processing software products for creating web, email, and print documents.
A .docx is an XML archive. You can extract its components the same way you open a .zip archive. Just change the file extension to .zip, double-click to open it, open the document.xml component and make the change Doug suggested. That part is fairly straightforward, but the next part is not.
I'm not sure how you re-archive the file; I asked Cortana how to create a .zip archive and got a lot of Web results; maybe Doug can help us there. But once you've achieved that, you change the file extension back from .zip to .docx and you're done.
Here's an article that tells how to extract the document.xml and then put it back; that's probably what Doug is talking about: https://www.groovypost.com/howto/howto/explore-the-contents-of-a-docx-file-in-windows-7/ (it's written for Windows 7, but the same thing will work in Windows 10).