Below is a simple schematic that was created to help show a possible way to use a DPDT switch to reverse the polarity on a motor. You can see when the switch is is the up position the positive (+) from the voltage source is routed to the positive (+) side of the motor. When in the down position the positive side(+) is routed to the negative (-) side of the motor.
Please note that this drawing is for informational purposes only. the full circuit would likely involve a form of protection and an off position in the switch. You also need to verify you have a bi directional motor. The Positive and negative on this motor are given for reference only.