To do this, please log in to your user account and navigate to the "My licenses" tab. Your active subscriptions will be listed there, along with an option to either edit payment details or cancel them, just click on the credit card icon next to your license.
Once you cancel your subscription, AdGuard will remain usable until the end of the current subscription period (e.g. if you purchased AdGuard in September but canceled your subscription in December, AdGuard would remain usable until September next year). From there, you will be able to either resubscribe or upgrade to a permanent license.