The quickest way to achieve this is to get a quote for an identical vehicle return journey and complete the booking form through to the basket page (with the T&C agreement checkbox).
Once here you can amend the vehicle type on any of the journeys in your basket by clicking the small icon next to the journey summary then clicking on "Back To Fares" on the left at the bottom of the page.
If you have any problems with this please contact our customer service team who will be able to assist.