One way I know is go to Start\Control Panel double click Date and Time,
select Internet Time tab and type in the name of the new time server.
(Example: tock.usno.navy.mil). Then click apply. There may be a registry
edit that you can make, but I have no ideal where it would be located.
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\DateTime\Servers
I believe you can enter only 3 time servers. When I add the 4th server, it
deletes one of the other servers. Hope this helps.