Assuming you are logged in to Pamoya Beta.
Navigate to your account page by clicking "My account" in the main menu. Your account details and, above them, a tab bar are shown.
On the tab bar click the "Edit" tab.
Just below it a second, secundary, tab bar appears. Click the "You" tab on that secundary tab bar.
To change your first name and surname, type your first name and your surname in the "First name" and "Surname" fields respectively.
At the bottom of the page click "Save".