In order to add or change credit card details for your Funnel subscription, navigate to https://app.funnel.io/#/billing/ directly or find the "Billing" option in your user menu dropdown:
Once there, the "Update credit card" should be visible to you. If it isn't, you are likely not the subscription owner. If possible, ask that user to log in to Funnel and enter the necessary credit card details.
If for some reason, you need to switch the subscription owner role from one Funnel user to another, please email firstname.lastname@example.org or get in touch with us via the in-app Support chat!