From the admin area of Kayako, click Account (or Plans, if you're a trial customer) at the bottom of the sidebar. Beneath the plan information, you'll see the 'Billing Information' heading. Here you can add or update your billing information and card details.
The system will automatically charge your card either once a year or once a month, depending on which payment plan you've selected.
NOTE: Our support team can't take customers' payment details over a call, chat, or email, so it's important to keep them up-to-date here.