When the alarm starts, it will trigger a notification.
Important to note that for for the notification to work, you should have authorised notifications for BetterSleep (Apps > BetterSleep > Notifications).
When you click on that alarm notification, it will open the alarm page as below
To turn the alarm OFF, click on Wake Up to turn it OFF or Snooze to snooze.
This will only turn OFF the alarm for today.
Important to note that the alarm will not start if you force closed the app fully and that if you can't open it through the notification, the only other way to do it is to manually open the app.
If you want to completely turn OFF the alarm, go to Profile > Alarm > Turn it OFF.