Bad customer service

I went to KFC Umgababa, only 1 self-service machine working. I went to the counter to place my order, I ordered 6 pieces meal ams and paid, but just after I placed my order, I requested to change order and order 9 pieces instead,, I was rejected and told that once order placed it cannot be changed. My order was not even being prepared yet. I received bad service and I asked to speak to the manager, she came and told me the same thing, to my knowledge, that is not official, I had the right to change my order.