How can I add (or update) Description Information in my support case?

I just opened a support case and afterwards I wanted to add some more details to the description. When I select the case in the Support Hub I can see all the details of the case, including the description, however, I only see the option to edit on Status, Priority, Subject, Product, Serial Number, Product Version, and Customer Reference. There doesn't seem to be an option to edit the Description Information field.

Any idea how I can edit the description field or add description information?

Thanks.