What resistor to use to drop 9volt's dc to 6 volt's dc?

To drop a 12 volt source to 6 volts with a resistor, you have to drop 6 volts. The value of the resistor you need would be 6 divided by the current the device pulls in amps.

For example, if the device pulls a half an amp the resistor has to be 6/0.5 or 12 ohms. As this device runs on 6 volts and draws 1/2 amp, it's wattage is 3 watts (volts x Amps). Common practice is to double this, or the resistor will probably get too hot and may open. I'd use a 10 watt to resistor to maintain a good margin for safety, and they're readily available.

Use a 12 ohm, 10 watt resistor.