"Ownership" is defined by the contents of the "whois" records, so you can effectively change ownership by simply editing the whois records (Registrant, Administrative Contact, etc.) from within the Control Panel.
Go to Control Panel -> Domains -> Registrations, select the check box for the domain you wish to change the owner for, and submit the form by clicking the "Modify Whois for Selected" submit button at the bottom of the page.
Edit the information on the resulting page(s), and the "owner" will now be as you have detailed with your edits.
Then, if you wish (you do not have to), you can "transfer" registration to a different registrar, renew it, etc.