Customers don't like changes to their appointment unless they are the ones requesting the change. Too much rescheduling or cancels is a common reason customers will leave low reviews, complain to support and simply choose to book someone instead of you.
That said, if you need to add or remove an extra, edit dimensions, change the service level, etc you can do so by editing the appointment.
For example, if a customer would like to add an oven cleaning to their appointment, you would go to that customer's appointment in your account, click edit appointment, click 'add option' and select the extras would like to add. In this example, just oven cleaning.
Remember, you must get the customer's approval BEFORE submitting changes. Failure to do so will result in a 1-star review. If possible, get the confirmation via text message so you can provide proof.