Como faz pra cancelar o plano Premium do App

if you subcribed via a mobile app store, contact that store.

if you subscribed via adobe.com, make sure you're aware of the cancellation terms by selecting your plan type (at the top of the page here), https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html

and for info on how to cancel your subscription, this page describes the few steps involved, https://helpx.adobe.com/manage-account/using/cancel-subscription.html

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative.

if you're unable to follow the steps described, check your account (https://account.adobe.com) for a payment problem. all payment problems must be resolved before cancellation is enabled, https://helpx.adobe.com/manage-account/kb/fix-failed-missed-payment.html