You can update any field associated with a contact (such as name, email address, birth date) on the Contacts page. Locate this page in the tab on the left-hand side.
Enter the email address of the contact you wish to edit and click Search.
If that contact is in your email database, it will appear in the search results. Click Edit next to the contact.
Click into the field you wish to edit and update the information. Make sure you click Save in the upper right hand corner when you're done, or your changes will not be saved.