Long time no see!
I don't think you can edit a scanned document in Word, as it scans as an image. How does Google claim to be able to do it?
What you could do is scan the document with an image editor, such as Adobe, then use the Text tool to replace the existing text.
I'm sure someone will come up with other suggestions, but I've occasionally spent loads of time trying to do something like this, when it would have been quicker to just type the damn thing out again from scratch.