If you have Uber credit on your account, it can be automatically applied to trip fares and cancellation fees. You can see how much credit you have on your account in the "Payment" section of the app menu.
When requesting a trip, you can toggle your credit on or off after you've set your destination in the "Where to?" field.
1. Tap the white bar displaying your payment method
2. Tap "Change" next to payment method currently selected
3. On the "Payment Options" screen, tap the toggle next to your credit amount to turn it on or off for this trip
NOTE: You cannot toggle your credit on or off once the trip is requested. To switch your payment method after a trip, tap the link below.