It's not possible to rename a mailbox once it's been created.
To get around this, you can create a new mailbox within MegabitCloud Web Hosting Control Panel and use the Email Migration tool to sync one mailbox to the other, then remove the original mailbox.
You'd use the hostname imap.yourdomain.com within the Email Migration tool and set the mailbox password of the new mailbox to match that of the old mailbox. You can then copy emails from one to the other.