If you need to change your booking's details (i.e. number of passengers and pieces of luggage, pickup time, etc.), you can do so by visiting your Trip Page and editing your transfer accordingly.
You can read the bellow article for a step-by-step guide on editing your booked transfer:
It is important for us know well in advance if you will be travelling with additional passengers, as this might mean that we need to assign to you a driver with a vehicle type that is different than the vehicle type required for your initial booking. For example a transfer with 3 passengers and 2 pieces of luggage would be assigned to a Sedan, but adding 2 passengers would require a Minivan to safely fit everyone.
Please note that a change that warrants an upgrade to a vehicle type with greater capacity, also means that the transfer price will be greater than that of the original booking, so extra fees will apply.