Your DNS server(s) are configured in the text-based configuration file that is created during initial system setup. To change it, you need to ssh into the Prime Infrastructure server (or access the console through the ESXi server hosting it) and modify the configuration file.
I don't have one on hand right at the moment but I believe you can re-run the setup script ("setup"). Alternatively you can "configure terminal" and remove the old and add the new dns server manually. "show run" will show you the expected syntax. End and write mem and the restart the server (ncs stop / ncs start)
