To change your account ownership/billing details, click the "Client information" link from the myCP main page (http://mycp.nanoservers.net/).
If you have the "Security ID" feature enabled (by default, this is active), you will be emailed a special code to your email address(es) on file that you will need to enter to complete the update. You should receive this email within a few minutes, but if you cant find it check your spam folder as it is an automated message.
In the event that you cannot receive the code, or if any further assistance is required, please open a support ticket in the billing category for further assistance.