Start with the fare breakdown on the trip in your ride history — it itemises the base fare, distance, any time or stop charge, a night charge, surge, waiting or delay charges, and any airport fee, tolls or parking, and those lines always add up to exactly what you were charged.
If it still looks wrong, raise it with us with the trip reference. We'll look at the recorded route, the timings and the tariff that applied. Where we've got it wrong we'll refund the difference; where the charge was correct we'll show you how it was worked out.