Hi there and welcome
you need to use an external USB hard disk for this.
I advise a 500GB one such as this one
But whichever one you choose it must have its own power source.
Then once connected to the WiiU you do the following
In Wii U Menu, press "System Settings."
Use the left analog to roll over to "Data Management" and click the "A" Button.
Didn't receive the instructions to format on start-up? Then you will have to tap “Delete All Data on USB Storage Device” and do the commands.
After you have formated your Hard Drive for Wii U, you should not try to format or partition the drive for anything else.(NTFS/Fat32) If you do the Wii U console will prompt you to format you Hard Drive again.
It is then ready for use and updates can be installed to it
If you have any problems, or would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.