Boot from Microsoft Windows Server 2012R2 DVD/ISO.
From the Windows Setup menu, click “Next”.
Select “Repair your computer”.
Click on “Troubleshoot”.
Under Advanced options, click “Command Prompt”.
At the command prompt, run the following commands:

c:
cd Windows\System32
ren Utilman.exe Utilman.exe.old
copy cmd.exe Utilman.exe

Close the command prompt, remove DVD/ISO and then reboot the system.
The server should now boot and present the logon screen.
Press Windows Key + U.
At the prompt you can change the password, by typing the following command:

net user administrator Password123

This will set the password for the Administrator user to be Password123 (case sensitive).

Now repeat the DVD/ISO boot steps and open command prompt above and run the following:

c:
cd Windows\System32
del Utilman.exe
ren Utilman.exe.old Utilman.exe
Done!