How do i Downgrade i Cloud extra space ?

I signed up for extra space on my Apple account for a year and have not used it . I received an email from Apple telling me if i did not Downgrade my account then i will charged every month for the extra space . I have tried on several of the Apple sites to find out how i do this but it seems to be much easier to sign up for than to downgrade !!! Thank you in advance for any advice on this .