To use your Uber Cash balance, you'll need to first need to set it as payment method. To do this:
1. Enter your destination
2. Tap your payment method
2. If you've setup a profile, tap "Change" under the profile you want to use
4. Tap "Uber Cash"
Your Uber Cash will automatically be used on your rides. If you don't have enough Uber Cash to cover your ride, you'll be given the option to buy just enough to cover your ride, or buy a set amount of Uber Cash and receive a discount on the purchase.