Everything to do with your subscription lives in Settings > Billing & Plans.
Your credit balance is shown there, and in the app header, so you can see how many credits you have remaining before starting something.
Upgrading takes effect immediately and the larger credit allocation is available right away.
Downgrading to a smaller plan is self-serve from the same panel and takes effect at the end of your current billing period, so you keep what you paid for until it ends. The panel shows what changes before you confirm.
Your payment method is updated from the same place. To update your billing card, open Billing & Plans and change the card on file, its expiry, or switch to a different one.
Cancelling is under Manage Subscription. See the cancellation article for what happens after.