If they are made for 14V they propably have the series resistor integrated in the LED. In that case I would say it is impossible to change it.
However you could try it at 12V, it might light up. At least it will not break.
If 12V is not enaugh you can put a 1,5V battery in series with your 12V power supply, the LED will work as long as there is energy in the battery.
You should also be abel to get 12V LED's from any electroinics store and then you can use the directly with your power supply. Price should be <1$ each.
Michael