To use Cash App Boost to get discounts on online payments.
Tap the Cash Card tab on your home screen.
Press Save with Boost.
Tap on a Boost.
Tap Add Boost.
Also, note most of the Cash App Boost works even for online transactions using your Cash App Card.
To change or swap the Cash app Boost, just go to the Cash Card tab on app and you will have the option to swap or change on the active boost.
Just tap on it and select any other boost you want to add.