This varies depending on the extension of the domain name.
If your domain is a UK domain (generally those registered for two years, such as .co.uk, .org.uk) then we can't make this change for you. When we registered the domain on your behalf you should have received an email from Nominet. You can use the email address you received this on to login to Nominet and update your details.
For most other domains we can make the change for you, simply raise a ticket and let us know.
NOTE: Please remember to update your details in our billing system as well if your address changes!