Contacts can be edited from the web user interface here.
Step 1: Once logged in, navigate to Contacts.
Step 2: Click on the Contact's name you want to edit.
Step 3: Click "EDIT".
Step 4: You can then change the Display Name, Address Field and Contact Emails/ Mobile Numbers. You can over ride the current Email/Mobile Numbers or click the + button and add a new contact to the existing Email/Phone numbers listed.
Step 5: Don't forget to press SAVE to update the contact details.
For more information on Contacts, read the Contacts Overview