You will still need a key to activate Windows, so this procedure is only useful if you went the evaluation route first.
Do this in an elevated command prompt:
dism /online /set-edition:ServerStandard /productkey:N69G4-B89J2-4G8F4-WWYCC-J464C /accepteula
Reference
http://woshub.com/how-to-upgrade-windows-server-2016-evaluation-to-full-version/