If you'd like to change billing, administrative and technical contacts, you may do it in Manager after logging in.
Then choose your domain in the menu on the left.
Select "Manage your contacts" in Management section.
A site will appear where you may change the contacts by clicking icon next to the contact name.
Then you'll get a site where you need to provide a new nic for that contact.
After editing the contacts, click "Accept" button.
You'll receive an information that your operation will be take into account in several minutes.