You could use a servo reversal cable ....but that is an expensive option.

I found this at helihobby Servo Reverser

*********************************************
All Helis

The Servo Reverser is a quick and easy way to reverse the motion of the servo and it is ideal for flyers who do not have a computerized transmitter.


9.99


PART#DD-SVRV-PKG-01

- Weight: 1 gram
- Size: 8mm x 44mm x 5mm (average)
- Power consumption: 4 mA