When you sign up the details should tell you that it is set to renew and it will renew at the same level, monthly or yearly, that you initially purchased.
You can cancel the renewal on your membership page and can request a refund by opening a ticket to support, if it renews and you forgot to cancel.
https://support.chess.com/customer/portal/emails/new (right now this is down but you can also e-mail support@chess.com or wait until that is back up).
Wondering if there are others who became a diamond member for a whole year involuntarily by forgetting to terminate the trial or whether it is only me who is forgetful. I think it is speculative that they tie the trial period up to a full year, a month of automatic membership should be enough.