The requirement you are looking for might not be possible because after embedding a dashboard image in a word document the interactive session is lost not only in word but also in a pdf, so I guess you cannot embed a view of a dashboard into a word document that refreshes.
Paramesh is correct - once the image is inserted, it becomes static.
One thing you could do is lean on Microsoft's web browser control inside the Word document. Essentially, you'd tell the web browser control to show a "live" rendering of the Tableau dashboard.
You should be able to take these instructions from Microsoft, and adapt them to Word (instead of PowerPoint):