I just read on the main GOG website that there is a major update to this game. What exactly must I do to get this update and how do I verify it is working once applied?

On GOG galaxy click on Witcher 3, at the top you should see a little symbol that looks like sliders. Click on this and then click on check for updates. If that doesn't work just uninstall the game then reinstall. On the main game screen it should say 4.0

Thanks!!