https://technet.microsoft.com/en-us/library/jj247582(v=ws.11).aspx
Transition to Windows Server 2012 Standard

After you have decided to transition from Windows Server 2012 Essentials to Windows Server 2012 Standard, complete these two steps:

  1. Purchase a license for Windows Server 2012 Standard and the appropriate number of user and/or device Client Access Licenses for your environment.You can purchase a license for Windows Server 2012 Standard from a Retail Outlet, a Distributor, or with the help of a Microsoft Partner.
    System_CAPS_noteNote
    If you purchased Windows Server 2012 Standard initially and exercised your downgrade rights to install one of your two virtual instances as Windows Server 2012 Essentials, you do not need to purchase anything additional.

    If you purchase Windows Server 2012 Standard through the Volume Licensing channel, you can download an ISO image and a product key for Windows Server 2012 Standard from the Volume Licensing Service Center (VLSC).

    If you purchase Windows Server 2012 Standard from all other channels can download an ISO image and an evaluation product key for Windows Server 2012 Essentials from the TechNet Evaluation Center. Performing the transition as described in the next step will convert the evaluation product to a fully licensed and supported product.
  2. Open Windows PowerShell as Administrator, and then run the following command.
    To know which edition you can change to, type dism /online /get-targetEditions

    dism /online /set-edition:ServerStandard /accepteula /productkey:    Product_Key
    Where Product Key is the product key for your copy of Windows Server 2012 Standard.

    The server restarts to finish the transition process.

    If your server is sitting behind the kms server, first find the “KMS client setup keys ” for your server edition https://docs.microsoft.com/en-us/windows-server/get-started/kmsclientkeys .

Then replace the product_key with the kms client keys. After that, do the kms activation with command:

Turn off Windows Server 2012 Essentials features

If you no longer need the Windows Server 2012 Essentials Dashboard or other value-add features to manage the server, you can turn the features off and remove them from your server.

The Turn off Windows Server 2012 Essentials Features Wizard helps you uninstall the features. It also cleans the server of files that were created by the Windows Server 2012 Essentials server software. Some cleaning operations are performed immediately, while others are initiated after the server restarts.

The Turn off Windows Server 2012 Essentials Features Wizard requires that you manually uninstall all add-ins before you can complete the wizard. To view a list of installed add-ins, open the Application page in the Dashboard. The wizard will alert you if it detects installed add-ins, and prompts you to uninstall them.

The Turn off Windows Server 2012 Essentials Features Wizard allows you to choose whether to keep backup files for client computers after turning off the Windows Server 2012 Essentials features.

There are two ways to run the Turn off Windows Server 2012 Essentials Features Wizard from the Dashboard: