Navigate to Customers > Contacts, find the contact you want to edit, and click the View button to edit it.
Note that the Default Email Address and Default Company fields are dropdowns. You won't be able to edit the email address or Company name on this screen. You can choose a different one from the list or click the Add New link to add a new one. If you need to edit the existing email address, you will need to go to Customers > Email Addresses to edit it.