That will depend on where you are seeing this card.
If you are for instance clicking or tapping on an "Apple Pay" button, the default card is stored in the Wallet app. Drag the cards up and down in the Wallet app on your iPhone to re-order them. The front most card will be se as the default card.
If you are paying through your account on a website, then the default card it chooses will be set in your account settings for that website.
If you are seeing the card as the AutoFill option when adding a card to a website, then it may be coming from Safari settings.
Go to Settings ➜ Safari ➜ AutoFill ➜ Save Cards and remove any you don't want, and set one you do want to use as default.