Unfortunately there is no easy way to replace your domain in the HELM Control Panel.
All child services (email, ftp, databases) are bound to your domain and cannot be moved to a new domain.
If you need to replace your domain you can first add the new domain and then delete the old domain, or delete the old domain and then add the new domain.
Please note, however, that by deleting the domain you also delete all files and accounts associated with that domain. We recommend backing up your files before performing this operation.