It's probably easier to take hogflyer's suggestion than it is to reprogram your servo, and cheaper than buying a Y harness and servo programmer. Simply plug your left aileron servo into the AIL connector on your receiver, plug your right aileron servo into the FLAP or AUX1 (whichever you have) connector, and change your model's wing type to "flaperon". That setting automatically changes your flap connector to an opposite aileron connector, and will also give you flaperon ability on your flaps switch but you don't need to (and probably don't want to) use that. You could also leave your wing type alone, and program a mix from the AIL to FLAP that reverses the inputs, and disables the flaps switch.

If you really don't want to do it that way, just buy yourself a "servo reversing Y adapter" which looks like a normal Y adapter which a bit of circuitry in the middle that reverses one of the servos. Still cheaper than a servo programmer.