Damage Item in the slot of Item GUI

The title explains it well.

I need to damage Item in the slot of Item that has GUI.

To put it simply, I have a medkit that can contain dressings, medicine(bottles) and syringes.
The dressings have durability. I already made the procedure work to check for specific slots and what
items they posses. The only thing I need to know is how to make the "deal damage" work.

I am aware that "get copy of itemstack" basically gets the copy of an item which is in the slot.
But that leaves me with question, how do I affect item's durability while it's in the slot.. directly?

If doing directly is not possible, how do I do it with the "copy" and then replace the original copy with damaged version?