In Hudway Pro mode there's a number of extra features available. You can learn more in PRO Mode features overview. To upgrade your status from basic to PRO, you need an Apple ID with a valid credit card linked. Here's what you should do:
1. Enter the Settings in the left-side menu.
2. Tap the Go PRO line.
3. On the subscriptions screen tap the desired subscription period.
4. The app will request you to connect to the App Store — you'll need to enter your Apple ID password.
5. You'll need to confirm your intention to make a purchase by tapping "Confirm" in a pop-up window you'll see.
6. After you've made the purchase, you'll see a date through what your subscription is valid, instead of the Buy button on the corresponding subscription screen.