You can update your credit card from your profile on the Web app, under the "Billing" tab. Here's a link that will take you there: https://app.zeplin.io/profile/billing
If your team is on the Organization plan, click the settings button on the top right of your organization dashboard to access the “Billing” tab and update the credit card.
☝️ Only those with an Admin role or higher can access the billing information