If it's your physical card, you'll receive a notification before it expires, which will let you know how much time you have until it expires. To renew it, you'll need to select "Renew Card" from the cards section and follow the in-app process, providing the details requested for your order. Each new physical card costs €9. If it is not renewed within the specified period, the card will be cancelled and you will need to issue a new one whenever you want.
If the expiry date of your physical card has passed and you have not renewed it, the card will be cancelled and you will need to issue a new one whenever you want. To renew your expired virtual card, you don't need to do anything yourself, as it will renew automatically!