To add (or change) your business name on your debit card, head over to your name and business name in the bottom-left of your Mercury web app. 

1. Select 'Settings' 
2. Scroll down to the 'Company Debit Card Name' (there will be a preview of what your card looks like beside this
3. Select 'Update' and change the name in the pop-up window

You're limited to 23-characters for your business name that appears on the card. Your new/edited business name will appear on all future cards issued on your account.