Download the package from Microsoft: https://www.microsoft.com/en-AU/download/details.aspx?id=7887 and Install.
Once installed you will need to turn on the feature.
Press Start > Control Panel > Programs and Features
On the left you will see the “Turn Windows Features On or Off”, click on that link.
Tick all the boxes below:
Press OK, then Active Directory Users and Computers will be installed.