Through the BIOS.
SpeedFan is a temperature monitoring program, not a fan controlling one.
OP--if you want to run your CPU fan at max speed all the time, disable thermal fan control in the BIOS (if there is the option).
If you really want to ensure the cpu fan is spinning at max, here's my suggestion: get a 3-pin fan-to-molex adapter, connect a different case fan to the cpu fan header on the mobo (because most mobo's need a fan connected to the cpu fan header to allow the computer to boot up), and connect the cpu fan to the molex connector. That will run the fan at max speed no problem. Or you could try using a different 3-pin fan header on the motherboard (make sure the cpu header is occupied though).