Whilst I agree with all who've said just get a servo reverser, there is another simple option. IF you can re site the servo a little so that the pushrod will connect properly to the opposite servo arm, it will give the result you want. I.e. if the pushrod is connected to right of the servo connect it to the left or vice versa.
Turning the tiller arm through 180 degrees will also do what you want, again you'll have to re mount the servo so it lines up properly.
To be frank, getting a reverser is probably simpler, things like this are best sorted whilst building and planning the layout but I'm sure there isn't a modeller among us who gets it right all the time. Me? I'm lazy, I just flick the servo reverse switch on my transmitter.
Chas