In order to cancel your subscription please sign into your account and either from the home page menu button in the upper left corner OR from within the Gantter editor itself under the Help menu there is a "Mange Subscription" option.

Once you click on the "Manage Subscription" click on the "Buy & Manage Subscriptions" option. 

Then click the "Reduce" button.  Then click on "Cancel Subscription" and agree to cancel.