You can only transfer ownership of a tado° device if you own it, and it is not rented.
If you own your tado° devices, here’s what you need to do to ensure that the new owners can use your devices:
Invite them to join your tado° Home via the app.
Once they have accepted your invitation, they can update their contact details by going to Settings > Home Details > Contact Details.
Next, you either need to remove yourself, or ask them to remove you from the tado° Home.