You can’t change the current rating of the switch. Plus you never need to lower the maximum allowed current for a specified switch unless you are looking to find a physically smaller switch.
To run the 12V LED at 24 V, you add a resistor in series with to the LED + terminal. The resistor value you need is found with Ohm’s law:
Ohms = Volts / Amps
You want to drop from 24 to 12 Volts so it becomes:
Ohms = (24 - 12)/ Amps = 12/Amps
I could not find a current rating for the LED on the data sheet so I can’t calculate the final answer for you. If this is a one off build you can measure the LED current when on @ 12 Volts and use that value to calculate the resistor value. If many units are to be modified it would be best to get the current rating from the manufacturer.
When you know the current and resistance then you can calculate the minimum wattage for the resistor using: Watts = Volts drop * Amps = 12 * Amps