typically, you would reverse a servo simply through your radio, but every now and again you need to reverse a servo physically in the servo itself, weather this be for flaps in a certain model with 2 servos connected together, or if you have mixed brands of servos, and don’t want to buy new ones for ailerons, or if you have a gyro and need to reverse the servo for it to stabilize the plane correctly, or whatever else you are doing, here is how you do it:
step one: get the servo you want to reverse, and unscrew it, being careful not to lose any gears or screws, it may be a good idea to simply tape the top of the servo, in order to prevent it from falling off when you unscrew the bottom (small cups or cans can be useful for keeping track of parts).
step two: find the positive and negative wires to both the motor and potentiometer, the motor should be pretty obvious, if the wires on the potentiometer are not color coded, then it is almost always, the 2 outermost wires, of the 3 that we care about (taking pictures before rewiring is advised so you have a reference to go back to in case you get the wiring confused).
step three: swap the positive and negative wires that connect to the motor and the two outer most wires of the potentiometer, doing this will make the servo rotate the opposite way, from the same stick input.
step four: reassemble the servo.
you are now done! The servo should function the same as it did, just with the servo rotating the opposite way of given stick inputs.
note, it is not possible to reverse all servos electronically like this, as rack and pinon servos like those included on Eflite umx planes, you cannot e rewired, and some servos, have their motor and potentiometer directly soldered to the circuit board, in which case although it is technically possible to reverse them, it is not really easy or practical to do, also I am unsure if this works on brushless servos, although in theory it should work.