To change the set of modifiers or combo elements in a dish with modifiers or a combo dish, you should click the button 
next to the dish in the "My Order" section, this corresponds to the “edit” action, i.e. you will be taken to the dish editor (configurator).
The button may look different.
Switching to the dish configurator when displaying selectors and dishes on different screens:
Switching to the dish configurator when displaying selectors and dishes on one screen:
Dish configurator: