The site provides a one-time 30-day money back guarantee.You can utilize that and then get what you really wanted. It is also possible the site will just refund the difference.
Either way, you should probably open a ticket: https://support.chess.com/customer/portal/emails/new
Hello, I just bought a year of diamond membership, my intention was to buy 1 month of membership. Is there possible to get any help regardless this issue?