In the Ria app, tap the Profile and settings icon, then tap "Payment methods" and "Add new payment method". You can then choose the payment method you want to add and follow the prompts to enter your payment details.
If you want to add a payment method online, log in and click "Account" on the left of screen. Next, click "Payment methods" and "Add payment method." You can then choose the payment method you want to add and follow the prompts to enter the relevant information.