You haven’t scrolled down to the bottom of the list. Self published custom DHs are there. Above that are alphabetical sorted ST provided device handlers.
When you selected the right DH then you will see where to edit, either in the IDE or the App.
I’ve thought you are using this DH.
Anyhow, you will find it on the bottom.
PS.:
I actually prefer this firmware, DH and SmartApp for my Sonoff/ESP based devices.