There is currently no way to edit the value of a "child option" in a cascading select field. It is possible to edit the parent list options, but not the child list options. This limitation is really a problem.
For the child list the only way to change a value is to delete it and then recreate it with a different text/name, and this will require manually changing all issues where this option has been used in order to update them with the correct new value.
Adding the ability to edit the option text will make it a lot easier to maintain such fields.