On the App, go to the relevant booking (view booking) then from the menu select ‘your vehicle’ and it should have an ‘edit’ button. It does not work if you booked via BA (or presumably other travel agents). It normally shows the full rental cost, even if you prepaid. And also, be careful if you have prepaid and select a cheaper car as it does not refund the difference, though you can use the credit towards extras such as topping up insurance. The alternative is to use the current cancellation flexibility and just make a new reservation!