If you are not paying for your subscription through iTunes, you must cancel your Fullscreen membership from a web browser. Log into Fullscreen.com on your computer and click on your profile picture in the top right-hand corner of the screen. From there, select “Settings,” then click on “Account”, and go to the section labeled “Your Plan”. You can deactivate your membership from there.
If you signed up through your iTunes account, then you will need to manage your subscription via your iTunes Store account. Please follow the instructions here to cancel your subscription.