Delete the installed update-data and install the latest update v33 or whatever its called.
You should be able to find it on the web if you look around a bit.
I had the same issue so I know it works

 

Go to settings, data management, internal, select the game, choose updata data and then delete.
I think thats hiow I did it.

But then you have to install the latest update for the "preparing update" to go away.

 

Hello, I've been having the same issue this morning! I just deleted my update file. The thing is, I'm about to install the same update file. I don't see the DLC update (EUR) anywhere on the internet, and that is definitely what my Wii U is trying to update. Anyway, I'll see if reinstalling v33 is the key, but I'm unsure....

Edit: 25 minutes later and after re-installing the update, I check out my wifi settings. Yep.. my brother created a new connection without DNS block (he didn't know). Anyway, if you run into this issue, data management > zelda > delete update file only > reinstall update. Make sure DNS settings are all set still (look for red X on data management). Any questions, feel free to quote me and ask.