I wanted to know how to make the program update the car information..for eg someone books a car for a specific day and later another person wants to book the same car for the same day i that, how do I make the program check and inform the second person that the car has already been booked for that day?
Keep a record of which cars have been booked for the day. You could do this in a car object, if you decide to have individual car objects, or just in some kind of list. When someone wants to book a car, look at the car (or list) to see if it's been booked already for that day.