Hi, please click the ⚙️ in the app, then choose "Manage Membership" to cancel. If you haven't used the app for a long time, the easiest way to visit the app is at
https://fullscreen-for-googletasks.com/ .
If the above doesn't work, please describe what happened when you tried it, meanwhile forward me the email receipt, and I'll cancel it manually for you. To find the email receipt, you can search "Receipt from Full Screen for Google Tasks" in your email.