Change Booking Email, Name, Phone Number
Head to your Bookings tab, click the booking in particular, then click Edit. From there you can change the email, name, and number.
Change Booking Quantity or Variant
Head to your Bookings tab, click the booking in particular, then reschedule. Then you can uncheck the email button (so the customer doesn't get another email, unless you prefer that), click 'Select a Custom Time', ensure that same start and end time is there, and change the quantity, and click 'Reschedule'
Although it's a reschedule this is the only way to update the quantity or variant. If you have to add or remove quantity, this is the only way to do so, however if you follow the steps above you can avoid the additional email to the customer and there would be no change to their time or booking ID.