Dudu is a rude cashier

Went to chicken licken then with my meal decided I would no longer like a half litre drink but would like a can, Dudu blew her top telling me that she wont change it because their systems are old then went on to say what was the point of her asking what I would like to drink. Never in my life have I been shouted at for wanting to change my drink, I am very disappointed at the way Dudu treats people