If you want to cancel the server, first make sure all data, backups, DNS changes, and application migrations have been handled. Once you are confident the service is no longer needed, follow the cancellation or non-renewal path available in your client area.
If your goal is only to avoid future billing, disabling automatic renewal or allowing the service not to renew may be enough. If you want the service fully closed, review the account options or contact support.
Do not cancel a live production server until you are certain nothing still depends on it.