You may need to do this if your payment method on file has expired or you want to change the card that's charged.
Sign in to the Console at https://sheets.tillerhq.com/auth/google
Scroll down to the Subscription area and click to expand it.
Click the "Manage Subscription" button to open the Billing portal.
Click "Add payment method" to add a new credit or debit card.
Enter the payment method details.
Click the "Use as default payment method" checkbox
Click the "Add" button.