Paso 1: Abres cmd (executar como administrador), then you type
dism /online /get-targeteditions
Paso 2: You copy and paste this code in cmd
If you want to upgrade to Windows Server 2016 Estándar que escribe:
dism /online /set-edition:serverstandard /productkey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /accepteula
Nota:
se ves "fallou a actualización", reinicias o teu Windows Server 2016. It will upgrade Windows Server 2016 Evaluation to Standard.