If you start a print or give a command to home and axis and the printer moves in the opposite direction that it should. Try reversing the wiring 180 degrees so that the colors are opposite.
For example, if your X axis motor wiring is plugged into "X-MOT" on the Printrboard and the colors are red/green/yellow/blue left to right. Unplug the motor wiring and plug it back in to X-MOT blue/yellow/green/red left to right.
Review Printrboard wiring diagrams here.
Also, be sure that you have the correct firmware loaded on your Printrbot. Some firmware is written to direct certain motors to turn backwards, depending on motor orientation for that model. For more on firmware, click here.