I've been wanting to rethink the interface for creating and editing rings for some time now.
Here's the best current work around (as of 2.0.16):
You can select the planet again, then add a ring, then click the 'X' delete button twice to remove the new ring and then the old ring. Then you can add a new ring to your liking.
(which is not an ideal solution)
How would you imagine that it might work? Or how would you like it to work?