1- Login into your Client Area at https://secure.plusplushosting.net
2- Go to the Domains --> My Domains section in the menu.
3- Click in the 3 dots icon and then in the Edit Contact Information link.
You can also edit the domain contact information on many domain names in one step, simply select the checkboxes in the domain names, once done, you will see many action options at the footer of the screen, select (in this case) the Edit Contact Information option.
4- Update/Edit or select from an existing contact, your domain contacts (Registrant, Admin, Technical and Billing) information, once done, click the Save Changes button.
Note: Based in the new ICANN policy if you update your Registrant contact information you will have to verify it to avoid your domain be suspension. For more details please check the "Registrant contact verification process and information" article in this KnowledgeBase.