In the Navigation Pane, click Grammar, and then click Natural Classes.
In the Natural Classes pane, select the natural class you will edit.
See Important below.
Edit the content in any of these fields:
Language Explorer will automatically create a natural class when you insert one or more phonological features in a phonological rule (Rule Formula field) or in one of the Affix Process Rule fields, if the resulting natural class is unique (does not already exist).
In this case:
The initial (default) name is "Created automatically for rule "<name>". You can type a new name in the Name field.
Regarding edits in the Phonological Features field in Natural Classes, it is recommended that you edit the field where the you inserted the phonological feature. A change here in Natural Classes may affect more rules than you intended.