Change nameservers at 1-2-3reg.co.uk

1. Login to your 123-reg control panel.
2. In the Domain names section, select the relevant domain name from the drop down box and click on the Manage button.
3. If the domain is a TLD i.e com/net/biz etc click on the Manage Domain Locking option. Click on the Unlock domain link and then click on the Back to Control Panel link.
4. Click on the Change Nameservers link.
5. Scroll down and enter your new nameserver records in the relevant boxes, then click on the Change Nameservers button.
6. Please note: If the IP addresses of your new nameservers are required, you will be prompted for them.
7. Scroll down and enter the IP addresses for the name servers in the relevant boxes.
8. Scroll down and then click on the Change Nameservers button.

http://www.123-reg.co.uk/support/answers/Domains/Domain-Configuration/how-to-add-private-nameservers-to-a-domain-name-1087/