I ran into this where the MSI package would not perform an upgrade, it said to uninstall first. But--I could not find an uninstall shortcut nor was it in add/remove programs. The solution is to run this:
MsiExec.exe /X{DBBC72B2-2442-4B8B-9D70-2ED6C0322916}
This will perform an uninstall so that you can install the beta.
If you have the tech preview it should be available under Add/Remove Programs under "VI Toolkit (for Windows)".
But when it's not--like in my case, msiexec will do the trick.