You can change or cancel your choice anytime before the choice deadline by navigating to the 'My Meals' tab. To the right-hand side of your selected dish, there is the option to 'Clear Order'. You can also select 'I'm Away' if you're no longer going to be in the office.
Once the choice deadline has passed, you won't be able to clear your meal choice. This is because the order has been finalised and may already be being prepared by the restaurant.