If you've already paid for a trip with a different payment method, but meant to use Uber Cash, follow these steps:
1. Tap the menu icon in the upper left corner of your app
2. Choose 'Your Trips'
3. Select the trip you want to switch the payment method on
4. Under 'Need help with this trip?', find the switch payment method box and tap 'Edit payment'
5. Check 'Switch Payment Method for the trip' and tap next
6. Select 'Uber Cash' and tap 'Next/Submit'