You can delete a linked debit card by logging in to your online account or through the app.
From your online account, click on your name in the upper right hand corner and select Settings. Scroll down to see the list of accounts. Select the account you want to delete to expose the Unlink button, and select it.
From the mobile app, select Settings (the gear icon in the upper left hand corner if you have an iPhone, or the option in the side menu if you have an Android), then "swipe left" on the account you want to delete to expose the Delete button, and select it.