This is done by logging in to http://cp.blacknight.com and going to:
Account (or Home) >> Registered Domains >> domain name >> Manage Nameservers
When changing nameservers, you need to allow for up to 24 hours for full DNS propagation.
Please note: This is *NOT* done under: Hosted Domains, Websites, or Manage DNS. In the above you can only set the domain to use "External Nameservers" to remove the domain from our nameservers. You can not specify what nameservers you want the domain to use here.
If this is a domain transfer please ensure you press the "Manage Nameservers" to ensure you are viewing the current nameservers as the system can be slow to refresh
When you are changing an IE domain's nameservers, you may be given an error as follows:
While checking Nameserver hostname for nameserver: NAMESERVER for domain: DOMAIN NAMESERVER is not authoritative for zone DOMAIN.
The error reported indicates that the DNS zone (the DNS information for the domain) is not present at the target servers - it would first need to be created there before you can amend the nameservers, where this is a requirement of the IEDR.
You would need to add hosting for the domain - at the new hosting location, or ask the providers of those nameservers to setup a zone file for the domain on their nameservers; once the zone is present, and the zone information is in-sync across all those servers, you will then be able to amend the nameservers for the domain to point to the new location.