How can I cancel my subscription?

I have been trying to cancel my membership. We only use it from May to sept how can I cancel to avoid charges 

You can cancel your subscription inside your Account's Billing page. From there, just pick the free plan and your account will be scheduled to be downgraded on the next billing cycle.

For your convenience, I manually cancelled your subscription now. You can also confirm this by navigating to the same billing page I linked above. And seeing that you won't be using the service anymore, I also issued a refund back to your PayPal account for the most recent charge incurred.