Go to "Manage Project" from the main navigation:

Then under the "Billing" tab, click add new payment method:

You'll then be prompted with a modal to enter an updated credit card to process your payments via Stripe:

Once you've added a valid credit card then it will appear under payment methods as an additional card. 

If you'd like to make this card your default then click the gear icon and select "Make Default":

If you would like to delete a credit card then click the trash icon and approve the delete action:

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