What to do:
After starting the update again, the download will continue where it left off. [How to]
If other devices or programs are using available bandwidth, or if your Internet connection has slowed, your connection with our services may time out. [How to]
Wireless interference between your Wii U and wireless router may be causing the download to time-out.
As an easy solution, try moving the Wii U into the same room as your wireless router during the update, and then moving the Wii U back once the update is complete.
If you are unable to do this, try our more extensive wireless interference troubleshooting. [How to]
Using an alternate DNS may improve your ability to complete the Wii U system update. [How to]
Still Not Resolved: