Canceling a Burner subscription is no different from canceling a subscription to any other app on your phone.
You can find instructions on how to manage your subscriptions here:
If you subscribed via the web, you may cancel your subscription by logging into app.burnerapp.com. From there, you can tap the cancellation link, follow the prompts that let you know about your grace period, and confirm. Once it's cancelled, you'll no longer see the cancellation link but will see a Burner expiration date as well as a renewal button in case you'd like to turn the renewal back on.
Keep in mind, for all subscirptions, cancellation does not immediately remove the subscription record. Subscriptions are paid up front, so they will live out the rest of their paid period. Once the paid period is over, the Burner has a 7 day grace period where you can extend it using credits if you need it. If you don't need it, you can let it expire and it will remove itself from your account.