You will notice a down-arrow sign in the top right corner; click that to open the dropdown menu. Click 'Settings & Privacy' > 'Settings' > 'Security and Login' > 'Change Password' > click 'Edit.' After this, enter your present password and then enter new password. Click 'Save Changes' to apply the change. If you also wonder how to change Facebook email, you will have to use the same dropdown menu and the 'Settings & Privacy' option to change your Facebook email address.