Editing custom fields in a MS Word document

One of my clients uses MS word with a custom toolbar which generates document templates (e.g. for reports) with custom fields on the cover page. These fields include document revision, client name etc.
On the client’s machines the fields are edited using the custom toolbar, which brings up a popup box with “document properties” containing the fields.
In LibreOffice Version: on Windows I can see the fields, and double clicking on a field brings up the “fields” dialogue box but I can’t edit the content.
How do I edit the content of the field without deleting the field? For example, I want to edit a document and mark it as the next version.

Additional info: Opening the .docx file in a file manager reveals that the custom fields are stored in “custom.xml” within the docProps folder. Editing this file manually then re-zipping the archive seems to work but is a laborious process.