Here is how to update your name servers at 123-reg.co.uk. Tutorial assumes you have a domain name registered through 123-reg.co.uk, and starts from their home page.
Now let's learn how to update our domain name's Nameserver settings
Click "Control Panel" at the top of the page, to sign in to your account.
Enter your username and password, then click Log In.
Select the domain name you wish to manage, then click "Manage".
Click the "Change Nameservers (DNS)" option under Advanced domain settings.
Enter your new nameservers, then click "Update".
That's it! The domain name's nameservers have been updated.
Please note that it could take up to 24 hours for your new nameservers to propagate and be recognized across the internet.