If you are on the 1.1.1.1 version of the app, you can turn on WARP with the "enable" button on the lower right of the screen. Or, tap the hamburger menu on the top right, then select "1.1.1.1 with WARP".

To subscribe to WARP+Unlimited, you can purchase a subscription. The price for the subscription is located on the button at the bottom of the 1.1.1.1 app home screen. Tapping that button will bring up a dialog for you to confirm your subscription. All payments are handled by the Google/Apple app store, and the payment information associated with your Google/Apple account will be charged for these subscriptions.