If you want to cancel maileo, first make sure you have a plan for any data, mailboxes, or domain DNS settings that still depend on the service. After that, follow the cancellation or non-renewal process available in your client area.
If the goal is only to stop future billing, disabling automatic renewal or allowing the service not to renew may be sufficient. If you want the service fully closed, review the account options shown to you or contact support for the proper process.
Do not cancel active production mail service until you are sure users and mail flow have been migrated.