Under field configuration I can see name of the fields and which screen they are used. Where can I actually edit that field itself; for example for a customer field, add a new customer name?

[Nic Brough -Adaptavist-]

Field configurations control how the field handles.

For actual content, you edit the issue.  For the values you can select from (for select type fields), you need to go to "admin -> custom fields" and change the configuration of that field.