On your Wii U menu, go to System Settings > Data Management > Pick the device where you downloaded your demo > Choose the demo you want to delete
I could have shown a video how to do it but it was so simple it doesn't justify making one.
Well I have learned one little trick throughout my years that has come in handy. If you have children, these demos are pretty nice to have. It gets them to the point where they do not play all that long, which is always a concern of mine. I have deleted most of them but I do like that have some in case the kids want to play.