Some activities allow you to leave an activity up until 12 hours before the start of the activity. Once signed up, the Join button will change to a Leave button, and you can click that to leave the activity and not be charged as long as it's outside of 12 hours leading into an activity. When you are joining an activity, activities you can withdraw from before 12 hours will show the message "Free cancellation allowed until 12 hours before this activity."
If it is within 12 hours leading into an activity, please reach out to your professional if plans change and you need to withdraw and/or receive a refund from your activity.