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.
To change your password, type the new password in the "Password" field. Confirm the password by retyping it in the "Confirm password" field.
Move to the bottom of the page and click "Save".