Change Server 2012 or Windows 8 Product Key

From the Start Screen (Modern UI) type

slui.exe 0x3

and press return

A new window should open allowing you to enter a Product Key

Install .NET 3.5 on Server 2012 or Windows 8, 10, 2012

Make sure you have the Windows 8, 10 or Server 2012 installation disc in your drive

In the example below, the installation disc is in the D:\ drive

Open an administrative PowerShell command window and enter the following:

Server 2012
Install-WindowsFeature –name NET-Framework-Core –source D:\sources\sxs

Windows 8, 10
Enable-WindowsOptionalFeature –online –featurename NetFx3 -source D:\sources\sxs

Wait until installation is complete, then run Windows Update to get the latest patches and fixes

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