To perform the transition (sometimes called "transmog"), open a command prompt on your server and enter in the following command.
Dism /Online /Set-Edition:ServerStandard /AcceptEula /ProductKey:<Product Key>
In our example, we are using a publicised KMS Client Setup Key. Remember to use your own product key. The transmog process takes several minutes. For more information about this process, you can see the following articles from TechNet;