This is accessed from the Administration drop-down menu. It displays a list of the current suppliers and customers and each has an icon at the right allowing you to edit or delete the relevant entry.
If you click Edit then you go to the same screen as when you entered the supplier or customer in the firsat place; just change the relevant fields (e.g. add or modify the address) and click Save to store your changes. Incidentally, if you change the name of the supplier or customer then any transactions that refer to that business will still refer to the same business using the new name.
If you wish to Delete an entry then the software will only allow you to do this if there are no transactions associated with that supplier or customer.