How to edit on BGA/PGA DECAL?
If you are trying to change the number of pins, you need to do that from the library (File->Library), for other changes (Solder mask, silk, pin shape, things like that) you can do it from within layout (Select the part, right click, edit decal), you will be offered the choice of changing just the single instance or every on your board instance. 

Note that doing it within the layout will not normally update the library copy.