How to reverse rotation direction of stepper motor

(Your bipolar 4 wire motor?) Current through 1a, 1b coils will turn the rotor so it is aligned as shown above. Turn on the current (in the appropriate direction) the rotor turns clockwise (half step) between 1a, 2a. Turn off the current to 1a, 1b and the rotor turns clockwise (another half step) to align with 2a, 2b. Reversing the direction of the current in the coils produces motion in the same direction. To change direction you need to reverse the current in one set of coils only. At the moment the two motors are connected in parallel and turn in the same direction. Swap ONE of the coil pairs over and you should be able to get the two motors turn in the opposite direction with the same drive signal.

Here's a timing diagram of a stepper motor:

As you can see, you have to do the following for one step:

1. Both clock lines are default high
2. Bring CLK A low
3. Bring CLK B low
4. Bring CLK A high
5. Bring CLK B high

When you want to step the other way around, you have to do this:

1. Both clock lines are default high
2. Bring CLK B low
3. Bring CLK A low
4. Bring CLK B high
5. Bring CLK A high

Therefore, you can just swap the clock lines. You can also invert the polarity of the lines (high <=> low). Unfortunately, I can't see anything specific to your setup because you didn't give specific information.

Also see this website for more information.