In order to change the NTP to point to a different source, you will need to modify the registry for the PDC emulator server.  Usually this is the first domain controller in your network assuming you didn’t change the FSMO roles for it.
Registry Change Location:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\Parameters
1. Change the ‘Type‘ key to ‘NTP‘
2. Add the ‘NtpServer‘ key [...]