First of all, just in case you are asking What are nameservers? Now that is out of the way.
To make DNS changes at 123-reg.co.uk, log in to your account and follow this guide.
1. Select the domain you wish to change and click ‘Modify Domain’
2. Click ‘Change Name Servers’ and enter your new name servers in the ‘Name Server 1 and 2’ fields.
3. Click ‘Change Name Servers’ to submit your changes.
Note: You need to enter a minimum of 2 nameservers.
Warning: Do not enter your IP address; this will cause problems.
Remember it can take up to 72 hours for the changes to take effect.