To change contact details in a domain, go to "Control Panel - Domains - Change admin-c":
Fill in the domain contacts and click on "Edit Contacts":
1) Fields marked with * are required fields.
2) "Publicity" is responsible for displaying/hiding your data in whois.
3) If the checkbox "Use this data in all contacts" is checked, the registrant's data will automatically be duplicated in the administrative and technical contact of the domain. If you need to specify other data there, then uncheck this box and you will have the opportunity to change these contacts.
4) Changing the domain registrant is tied to the security level that is set in your control panel.
5) After clicking on "Change contacts", the changes will be made within 5 minutes.
6) You cannot change domain contacts if the block on transfer to another registrar is lifted.