It is unfortunately not possible to simply exchange tickets. You can ask the organiser to refund your ticket and make a new order. A link to contacting the event organiser can be found just beneath the main picture on the event page itself.
The organiser's email address can also be found in the confirmation email.
Remember to include the 7 or 8 digit Order IDs for all of your purchases (these can be found on your tickets and in the confirmation mails), as these will help us or the organiser to more quickly process your refund.
Note: Your refund may take up to 21 working days to appear in your bank account. The booking fees and services are not refunded