If the card is a DEBIT card, it is directly linked to the account you have associated with it. The amount available on the card is directly connected to the balance of the linked IBAN. You don't need to worry about topping up in this case.
If the card is PRE-PAID, please follow these steps to top up:
Click on the + (top-up) button underneath the card. A Transfer Money pop-up will appear. Select the account or card from which you want to transfer money. Select the prepaid card as the recipient. Confirm. Select the amount. Review the transaction and confirm. Please note that the account or card from which the funds are transferred must have a positive balance.