Using the ESX image for OwnCloud how can you change the default website to be the OwnCloud instead of the Univention Portal?
I don't think you can. ownCloud is an app on the univention server.
Also, could you clarify what you mean by default website?
Do you mean that if you type in the IP address or the domain name, that you get ownCloud instead of the univention portal?
Here are some posts from the univention forum. You can do what you want to do with a reverse proxy server.