What you need is a resistor with 50 ohms 5% tolerance @ 2.5 watt, cut the black wire and connect each end of the resistor and that's it. Icreasing the resistance value will decrease the fan speed, but will not see the difference unless the change is by the hundreds. This is base on 230 miliamp of fan.
Or like einstein4pres suggestion use 5 volts.
The yellow wire is 12 volts, the red wire is 5 volts and the two black wires are ground. So all you need to do is extract the pin from the molex that goes to the fan and move it to the other end of the slot where the red wire go.
Note that if you have slave wire connected to the fan molex connector, the voltage will also be 5 volts positive so DO NOT connect the molex back to the power supply terminal or you will short the wire and also make sure that you do not reverse the polarity of the wire.