Once you submit your request for the activity you'd like to book, your activities vendor will receive the request. Assuming, the activity is not available in their booking calendar, you will receive a "Reschedule Activity" email who will ask you to select another date and time.
In that email request to change, you will have the ability to click on one of the many dates and times that fits best with your schedule.
Otherwise, if you already scheduled your activity, and you'd like to make a change to that activity, you would need to call up the vendor directly, and request another date and time.
Once you activity has been confirmed by your activities vendor the first time around, your credit card will be charged, therefore, if you need to make a new date change, there is no need to submit another payment