I'm writing a book. I uploaded the file from my computer to Google Docs and shared it, but I want to update it by re-loading the computer file onto the Google Doc, overwriting it. How can I do this?

Google drive will accept mutable files with the same file name. This would make it hard to figure out where you were. Appending the name to include a sequence number or date may help and give you the ability to roll back to an earlier version if need be.