In Automatic Pro you can log your fill ups and set a custom price for the amount you paid for gas, the number of gallons, and the date, time, and location. You do this by navigating to Health > Fill ups > "ADD FILL UP".
To change the default and auto-filled entries, tap in the field you want to change and start typing.
To edit a fill-up after the fact, tap on any previous fill-up, then tap "EDIT FILL UP". Tap through the screens until you reach any parts you want to edit and then tap on the data that's there to change it.