Instructions
Unplug the Wii to cut power to it. You can do this while the Wii is powered on or powered off. Removing the AC adapter from the wall outlet is best when the console is powered down, but if your Wii has locked up, you may not be able to turn it off manually.
Count to 10, and plug the cord back into the wall outlet. This should restore power to the Nintendo Wii console.
Push the console's power button to turn the console on.
Run the system update again.