Upgrade Windows 2012 Evaluation with a full version key

Open an administrative command prompt

Use
DISM /online /Get-TargetEditions
to list the versions you can upgrade to. They are listed under "Target Edition"

To upgrade from Evaluation to Standard use
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula

You should be prompted to reboot.

This will not work for Evaluation Domain Controllers. See here