All depends on the aircraft, the catering carts, and the length of the delay.
If it is long enough, the food is all tossed and the aircraft is recatered. If the new schedule can make even remote sense for the food catered and the catering is not "past" it can be repurposed.
Carriers watch this carefully, particularly when it is warm and the aircraft will be without shore power or AC over night. Other aircraft are set up for return catering and that may mean that the food can be stored a bit longer (usually insulation).