On the Payment screen in the app, you are able to toggle Balance Auto Update on and off. Below you can see the toggle in purple when it is on, it will be white when it is off.
The Balance Auto Update amount you selected will load onto your account as credit balance that will apply to all your rides. That amount will automatically reload your balance from your default payment method whenever your current balance reaches $0.
You will need to enable Balance Auto Update to take rides once your balance reaches $0. When you reenable you will be able to reselect your Balance Auto Update amount.
If you wish to prevent further reloads, simply toggle Balance Auto Update off. When your balance reaches $0 you can then toggle back on, select an amount to load, and toggle back off to prevent automatic reloads.
To change your Balance Auto Update amount you simply need to toggle off and then on again. You may need to have a $0 balance when you switch the toggle for the selection screen to appear.