If timing group is already added, you can edit timing group to change name, to make it active in-active or add holidays timings.
Admin Portal >> Operation >> Advance Operation Setup
Open Timing group overview
Find the timing group which you want to edit from existing group list
once found, tap on “Edit” button.
It will open a form to edit timing group details.
Checkout how to add timing groups
Checkout how to add or edit special date at timing group