To edit the contact details of a customer or supplier go to Customers > Customer or Suppliers > Supplier and click the yellow edit icon next to the relevant customer/supplier
New Customers or Suppliers can be added from each data entry screen as you need them or by using the New option on this screen. They can also be imported from a csv file. See Importing Customers and Suppliers.
When you add a customer or supplier there are two tabs of information that need to be entered.
The basic details tab lets you view their details including their default category. If you no longer want to use them they can be marked as inactive:
The second tab will include their contact details as follows: