You haven't started the trial period:
Tap "X" in the top right corner on the payment screen. Your trial will start instantly — with no commitment to pay in 7 days.
You’ve already started the trial period:
Cancel the subscription in the Settings (iOS) or the Google Play Store, as explained above (refer to the item 'You want to cancel your subscription').
Please note: if not canceled properly, the subscription is seen as active (and subject to recurring payment) — even after you delete the app. This applies to all apps, not just Endel.