The first thing you need to do is determine your currently installed version. From an elevated command prompt, run the following command:
DISM /online /Get-CurrentEdition
You will see something similar to the following:
C:\Users\Administrator>DISM /online /Get-CurrentEdition Deployment Image Servicing and Management tool Version: 6.2.9200.16384 Image Version: 6.2.9200.16384 Current edition is: Current Edition : ServerStandardEval The operation completed successfully. C:\Users\Administrator>
Look for the line that starts with “Current Edition:” and take note of the exact text minus the Eval at the end. It will either say ServerStandardEval or ServerDatacenterEval.
Now with your license key in hand, run the following command:
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
Reboot the server when prompted.
When it comes back up, you will notice that the text in the bottom-right corner of the wallpaper no longer says evaluation. Go to the Control Panel, System and activate your server. It should all go through smoothly and be ready to go now.