If you have an active subscription made through Apple, you can find your subscriptions here:
Settings > Apple ID > Subscriptions
Tap on the subscription you want to cancel and select Cancel Subscription at the bottom. If you do NOT see this button, it is because your subscription has already been cancelled. If you have just paid for a new period, you will still have access to the service until it expires.
You will also be able to see a text, for example "Expires 03/01/2023"
If you have an active iCloud+ subscription, see how to downgrade here:
If you want to change your payment method, see here:
If you have regretted your purchase, you can request a refund, see more here: