This is for if you are an owner of the server.
Main >> Networking Setup >> Nameserver IPs. Here if the nameservers or IP's are not assigned then please assign using the option below.
If you are already having an IP assigned then first Cpanel will update those and if you wanted to EDIT the existing one then you have an option (X) using which you can change it.
This is for if you are only having an shared account on server.
Please contact your domain sponsoring registrar asking them for changing the nameserver.
Please let me know if you need any sort of more help.