1) You can "Manage Contacts" and add a "New Contact" by using the application menu drop-down at the top-left of the website
2) You can also edit a selected conversation's contact information directly by clicking the drop-down next to the desired conversation, and clicking "Edit Contact"
3) Upon selecting either option, the contact window will appear, allowing you to type in new information, or search for and choose an existing contact to edit.
4) When you're done editing the contact's information, click "Save." Navigating away to a different contact before clicking "Save" will result in a loss of changes.