Owners data have to be posted in a public database (Whois) managed by the competent authority for each extension. At the registration order moment the customer accepts the responsability of the communicated data.
It's possible to protect your data activating the "privacy protection" service. Activation may be required in the registration/transfering procedure of a domain name or in the control panel.
If you want to change the property of a domain name (not just data changing) you have to pay € 7.00 + vat for each domain, if the modification is just for some data (telephone number, email, address) the modification is free.