From the Cards screen (second icon on the Vault section of the app), find the card that was lost or stolen. At the top right, you should see a vertical ellipses (three dots). Tap these three dots and select Report Lost or Report Stolen based on your situation.
*Please note that this action is permanent and irreversible. You will need to order another card if you wish to have access to a physical card. Replacement cards cost $4.00 and will be debited from your account.*