Using the Active Directory Domain Services Configuration Wizard

The Active Directory Domain Services Configuration Wizard can be used to forcefully demote a domain controller when the Windows Server installation is still bootable and you are able to sign into it with administrative credentials.

Follow these steps:

1. Open Server Manager, (servermanager.exe).
2. In the gray top bar of Server Manager, click Manage.
3. Select Remove Server Roles and Features from the menu.
4. On the Before you begin screen, click Next >.
5. On the Select destination server screen, select the local Windows Server installation from the server pool list.
6. Click Next >.
7. On the Select server roles screen, deselect the Active Directory Domain Services role from the list of installed roles.
8. In the popup screen, click the Remove features button to Remove features that are required for Active Directory Domain Services.

1. On the Validation Results screen, follow the demote this domain controller link to acknowledge that the domain controller needs to be demoted before the Active Directory Domain Services role can be removed. 
2. On the Credentials screen, elect the option Force the removal of this domain controller:

1. Click Next >.
2. On the Removal options screen, select the Proceed with removal option and click Next >.
3. On the New administrator password screen, enter the new password for the built-in administrator account.
4. Click Next > to proceed to the next screen.
5. On the Review Options screen, click Demote.
6. When configuration of the Active Directory Domain Services server role is done, click Close to close the Remove Roles and Features Wizard.