Sometimes it's necessary to have a servo that turns in a direction opposite of the other servos you're using. I needed to do this for one of the servos that will raise and lower the "fans" on the side of the helmet since that servo is flipped around. If I didn't do this then one fan would rotate upward while the other one rotated downward.
There are two ways to reverse a servo's rotation: buy a special connecting cable or modify the wiring in the servo. I chose to modify the wiring. These instructons apply to the Hitec HS 300 servo but most servos are similar in construction.
1- Remove the screw holding the servo wheel/bellcrank
2- Remove the screws holding the servo case together
3- Remove the bottom portion of the servo case, exposing the drive motor
4- Reverse the position of the two motor wires
5- Remove top of servo case, paying attention to the gears and their positioning
6- Remove the retaining nut for the potentiometer
7- Remove circuit board from bottom of servo
8- Remove potentiometer
9- There are three wires connected to the potentiometer- swap the two end wires. Do not touch the center wire
10- Reassemble servo