Step-by-Step upgrading 2012R2 domain controller to Windows Server Technical Preview domain controller.
– Start by taking a backup of your server. – Insert DVD and start a elavated command prompt and goto the “support\adprep” folder. – Run the following command “adprep.exe /forestprep” This will upgrade the schema to version 72. – Run setup.exe from the DVD and choose to download updates when asked. – Choose to do an upgrade of the server. – The upgrade will now take place. The Server will restart 4-5 times during the upgrade process.