The fuel price is updated every time you enter a fill up. The default price is $1 (or 1 of your currency).
To do a fuel fill up on the iOS app, press the fuel symbol at the top right of the home screen. Android users will have to wait a few weeks until we release this (we are working on it at the moment)
Select the date, enter the amount of fuel, and the total price paid. The app calculates the price per litre or gallon. Then press 'Add Fill Up' and the fill up will be added to your timeline and the price for all your trip from now on will be updated.