If you are not happy with the fuel policy on your booking, you can easily change to a different option by logging into the Manage Booking section of their website. As the fuel policy is tied to the specific booking chosen, you will need to select a different option to change the fuel policy. If you click Change Vehicle this will give you the option to run the search again, and you can change to an option with different fuel policies.
If the price of the new option is higher or lower than your original booking, only the difference will be charged or refunded.