You can update your payment details (including credit cards) by logging into your Pusher account at https://app.pusher.com
You'll need the account details that were originally used to create the account, so if you're not the account owner you may need them to do this for you.
- Click on "Plan & Billing" in the top right corner
- Click the "Payment Details" button on the new page under "Update your billing info" section
If you can't do this or have further problems, please log a support ticket using the link below.