You can change your payment method before and during a trip. Please note that if you are in a city that accepts cash as a payment option, you will not be able to switch to or from cash while your trip is in progress.
While on a trip, swipe up on the app to change your payment method. Tap the last 4 digits of your card number to go to the Payments section of the menu. There, you can select a different payment method already linked to your Uber account or add a new payment method.