Thorsten,
You can find the directions here:
http://docs.aerohive.com/330000/docs/help/english/ng/Content/reference/virtual-appliance-release-notes.htm
Scroll down to "Upgrading from other HiveManager Virtual Appliance versions" and select the first one.
In short you must deploy a new HaveManager that is version 12. Then install the 11.26.3.6 patch to prepare you old system. Export the configuration and then import it to the new HiveManager. I had to do it about a month ago. Mine was very small (one switch) as I am just starting to deploy my switches and my APs are still on classic, so it went really fast. However the bigger your system the longer it will take.
