Reversing the DC polarity to a DC motor is a relatively simple process using a DPDT switch. I tried to load an image link but am having problems. The below should give you an idea of a few things to consider.
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However, things may not be quite as simple as the switch circuit makes it look. Small box fans as frequently found in home computers can't be reversed. They are not designed to run in reverse and when connected with reverse polarity will not run at all or some may emit smoke. Fans like this are designed to move air in one direction, right down to the blade pitch of the blades. Most fans like this are designed to run on a full 12 Volts or PWM (Pulse Width Modulation) which precludes using reverse polarity.
The first thing you need to do is determine if the fan in question can run on reverse polarity. Even if it does you need to determine if it will move air when running reverse polarity.
What exactly are you trying to do? Based on your post I assumed a DC motor?
Ron