To point a domain registered with 123-reg to your hosting account, replace its current nameservers with the nameserver details provided in your hosting welcome email.
Visit the 123-reg website and open the Control Panel.
Sign in to your 123-reg account.
Select the domain you want to update, then click Manage.
Scroll to Advanced domain settings and choose Change Nameservers (DNS).
Enter the nameserver addresses supplied by your hosting provider in the available fields, then click Update.
Allow time for the nameserver changes to propagate. The time required can vary.