Is there a way to edit a existing design group?
Is it plan to support this?
Please explain better so that we can understand what you actually mean. Use exact terminology, too – KiCad doesn’t use the word combination “design group” as far as I can remember.
Yes, you are right.
The correct terminology is “design block” (my mistake)
I created some specific blocks in a library. Now I’m looking how to change details without creating a new design block.
Have you tried to save with the same name?
You can overwrite an existing design block, or delete it and create a new one with the same name. Design blocks are pretty new and a lot more features are coming in v10. (PCB design blocks, updating them, etc.)