You should check if your activity is changeable or not in the “About this experience” section of the description.
If it is changeable, how to change it can vary. Certain activity providers allow you to change the date and/or time of your booking online by including a “Reschedule my booking” link at the end of your confirmation email. Others require that you contact them, using the contact details on your booking voucher.
Once you’ve booked your activity, you’ll get all the details by email directly from the activity provider, together with the booking voucher containing their contact details. They will detail procedures for making any changes to the booking. They will also use your email address to communicate directly with you about meeting points, campsite pick-ups and any other logistical details, coming up to the date of the activity.