Your subscription can only be cancelled by you. Once logged in you can do this at any time by going to https://www.busuu.com/dashboard#/settings/subscription and then clicking on "Cancel Membership". If this isn't done, your membership will be renewed automatically.
If you have paid via PayPal, you will need to cancel the recurring payment with them.
Log in to your PayPal account.
Click Settings (cog-wheel icon)
Go to Payment Settings
Click "Pre-approved payments".
Find the busuu Limited subscription in the list
Click Cancel Subscription in the top left-hand corner.
Of course you will still be able to enjoy all the Premium advantages until the end of the period you signed up for.