Firstly you need to log on to your account and access the "My Domains" tab;
Then you need to click the expand button of the domain you wish to add new contact details to;
On the following page you need to scroll down to the bottom until you see Management Tools. Here you will find the "Contact Information" button, like highlighted below. You need to click this to go to the following page where you will be able to edit the contact details of your chosen domain.