See the articles about Document Properties in this MVP FAQ index page:
Word Macros and Visual Basic for Applications (VBA) FAQ

You are first updating the properties. Are you sure you need to update the fields? That will require opening each file.

Here are links to two free utilities for batch handling files. You can use them to run your code.

• Document Batch Processes by Graham Mayor, MVP
• Batch Folder Process Add-In by Greg Maxey