If this is the first time you are updating your system:

The update can take up to an hour to download. When updating a new Wii U, the system update may take an extended amount of time to download. Depending on the speed of the Internet connection used, it could take an hour or more to download and install. Please be patient.

During the update, the progress bar displayed on the Wii U GamePad may take a while to start moving. This is normal.

Do not unplug your system while the update is downloading or installing. If the system update is interrupted by unplugging the system, you may damage the system.

If you have downloaded the previous system update (before December 4) and are now downloading the latest update:

If your system is connected to the Internet, it will automatically download the system update in the background, which allows you to use your system while the update is downloading. You can check the status of the new download through the Download Management screen.

Depending on the size of the update, it can take up to an hour to download. New system updates may take an extended amount of time to download, even if you have already performed a previous update.
If the download is happening in the background, it is okay to power off the system using the power buttons of the system or the GamePad. The update will resume when the system is powered back on.

Do not unplug the system while the update is downloading or installing. If the system update is interrupted by unplugging the system, you may damage the system.