Disconnect your trackpad, by pressing the power button until the green light goes off.
Type the following command in your Terminal, all on one line: sudo defaults write com.apple.MultitouchSupport ForceAutoOrientation YES
invert your trackpad, turn it on, then place five fingers on it, stretched enough that the device can recognise a hand shape. Congratulations, your trackpad is now inverted.
If you want to go back to the previous orientation, just switch the trackpad off, turn it around, and again place five fingers on it on reconnect.
