A premium membership is billed as a subscription. You can cancel at any time, but no later than four weeks in advance.
At the following link you can submit your cancellation and get all further information about your membership.
https://www.neuronation.com/subscribe/manage
If you are not logged in, you will be prompted to log in with your email address and the appropriate password.
If you forget your password, you can request a new one and follow the link again.