Unfortunately, it’s not possible to change user's email address on Conrad Connect Platform.
However, you can always create a new account. To make a migration easier, there is an option to publish current projects, create a new account, connect devices there and import published projects. Also, please do the "Revoke access and delete data" procedure on the device page in the Devices & Services section, and then you can connect same devices to a new Conrad Connect account. The old account could be deleted afterwards.
Since June 2018 Conrad Connect has introduced new services and features available on Service Marketplace. Previously created accounts have remained their preferences in gratitude to the first users, although accounts created after June 2018 have different account preferences. However, Conrad Connect platform is still open to everyone and free to use.