You can cancel your Apple Card if it is lost or stolen by contacting an Apple Specialist at 1-877-255-5923 or by navigating to the Wallet app on your device and opening your digital card. Tap the 3 dots in a circle in the upper right hand corner then tap the Message button to chat with a specialist.
You can lock your card by following these steps.
To request a replacement open your Apple Card in the Wallet app and go to the More screen (3 dots in circle), then scroll down to the Physical Card section and tap Request Replacement Card. You will then be sent a physical replacement to the address linked to your card. Scroll down on the Apple Card screen to make sure your billing address is correct.