In /core/local.config.php - change the fuel price.
Config::Set('FUEL_GET_LIVE_PRICE', false); // live fuel pricing does not presently work
Config::Set('FUEL_DEFAULT_PRICE', '.83'); // this is price per pound in USD - change accordingly if you use a different unit or monetary measurement
Config::Set('FUEL_SURCHARGE', '5'); // this is the price surcharge in percent for non-hub airports - 5% more than a hub airport