To change the owner, login to the Control Panel and navigate to the Domain -> Registrations screen. Once there select the "Check Box" under the "Modify Whois" column (second column from the left, immediately adjacent to the domain name) for the domain you for which yhou wish to change the ownership information, and select the "Modify Whois for selected" button at the bottom of the page.
On the next screen, just select the contacts for whom you want to enter new information (since you are "transferring ownership", make sure you select the "Owner" if nothing else!), under the "Whois Preference" Section select the Radio Button for "Or, use your own contact info:", enter the NEW owner's information where indicated, and select the "update contact info for 'yourdomain.tld'".
That's it - the New owner now "owns" the domain.