If the server is a domain controller, you cannot convert it to a retail version. In this case, install an additional domain controller on a server that runs a retail version and remove AD DS from the domain controller that runs on the evaluation version. For more information, see http://technet.microsoft.com/en-us/library/hh994618.aspx.
To convert edition do next steps:
1. First, check version. From an elevated command prompt, run
evaluation versions will include “EVAL” in the output.
2. To see editions available for us to convert, type
DISM /Online /Get-TargetEditions
You will get a list of edition IDs which we will use later.
3. Convertation itself. Run from elevated cmd:
DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
providing the edition ID one from the list we got above and a your retail product key. The server will offer to restart (twice) and all will be done.