Microsoft encourages Window users to always run the updates when one appears on your screen. It is generally in the best interest of the user and will make the system run better and more efficiently. If you receive Error Code 646 while applying the update, make sure first you have internet connectivity. Next, it may be the Microsoft server program that you can’t connect with. Finally, check your firewall and make sure that it’s not blocking the connection. If all else fails, go to Microsoft’s website and there you can find the answer to how to run the update.