Assuming you disnt generate the code from a CAM system (in which case mirroring in the CAM system and reposting would be easiest), and assuming your control wont do it... Use an editor such as cimco, editcnc (you can search for free downloads or 30 day trial versions of these editors) they have a function that does this automaticaly.
If you are simply miroring about the X axis, all non-zero X values and I values get the sign changed from positive to negative or vice versa. The editors I mentioned also have a backplot feature that will display graphicaly the edited toolpath.
Try this link
http://www.editcnc.com/cnc-programming.html
CAM teh "left ... left ... left . right . left"