If the spindle has a dc motor, just revers the pos and neg wires.
If it is 3 phase ac, just reverse any 2 leads. This is assuming that you are using M03 to turn the spindle on and off with the g code.
I would change the way the spindle is wired as it will make programing the same from machine to machine and program to program. This all assumes that the motor is setup to reverse direction from mach3 using the m03 and m04 commands.