One way is to get some power diodes and connect them in series with the output.
A general silicon diode drops approx. 0.6 - 0.7 volts across it. Therefore to drop 6 volts you'd have to connect 10 of them in series.
Next you have to consider the rated power dissipation of the individual diodes. Find what the maximum amps will be that your power supply will ever supply and use that. Power = volts x amps so let's say the max amps from your power supply would be 8 amps then 8 amps x 0.7 volts = 5.6 watts. You would need at LEAST that power rating for each diode, but preferably higher for a good safety margin.
I just happened to be buying some 5 watt diodes yesterday and they were priced at $1.65 each. However, those prices are high and you'll probably get them cheaper on Ebay, etc, or if you buy from China, a fraction of that price.