Debit/credit cards show the digits of the credit card on the front side of the card, that's why they end up being used more commonly. The debit card digits are however found on the bottom of the other side of the card. You can also find the debit card's CSV number on that side of the card.
If you have been using your credit details as your payment method but would prefer to use the debit card details instead, just remove your old card information under "Payment methods" in the app and type in the debit card details in the payment card section, and voilà! You're good to go!