Or if your radio doesn't offer reversing, just buy a reversed travel servo or open up a standard servos and modify it for reverse travel.
Buying a reverse servo would be better than going inside and making a normal travel servo into a reverse servo, unless you know what you are doing. I have done that and it's only possible if you don't have a "surface mount" servo. With a surface mount servo, you don't have any wires running from the pot to the motor or amp.....
Larry/Instructor