the readme Text in the Upgrade package is not correct , because it runs in that error that you cant upgrade because it is the normal install process.
Once the files are uploaded, your XenForo installation will automatically be closed until the upgrade is complete. To begin the upgrade, go to the URL that corresponds to <your XenForo Directory>/install (eg: http://www.example.com/xenforo/install). You will be instructed to login as an administrator and guided through the steps. Once the upgrade is complete, your XenForo installation will automatically be reopened.
Upgrading works with : http://www.example.com/install/index.php?upgrade
And that works like a peace of candy