Here is another question I answered from the now defunct Makers StackExchange.

Question:

I have a Prusa Mendel built from a kit from Makergear.

The way their parts are set up, the X-axis is reversed from normal. This doesn’t make any difference for most prints that are organic or symmetric, but it means that printing many parts come out “mirrored”. For example, printing replacement parts for my own printer is tedious because I have to flip the STL models by hand before printing. So basically the question: Is there a way to print correctly on a machine with a flipped X-axis that doesn’t require fliping each STL model before slicing? My guess is there may be a way to either tell the slicer or the firmware to take the mirroring into account, but I’m not sure what would be the right approach.

My Answer:

You can repair this by updating your firmware. If you are using the Marlin or Sprinter firmware, you can adjust the setting

define INVERT_X_DIR true

In configuration.h and use that to update your electronics, see this blog post; for a guide on how to update your electronics with the marlin firmware (Assuming you are using a RAMPS setup).