Electronic devices typically operate at lower voltages than the one at which electricity is supplied, so they may have internal circuit components that step down the source voltage. If you have a device lacks this type of internal voltage protection, you can supply it by constructing an external resistive voltage divider circuit. It's possible to step 12 volts down to 6 volts by incorporating a pair of 10,000-ohm resistors into the circuit.
Cut two lengths of wire, and strip each wire of 1/2 inch of insulation at each end. Attach one end of the first wire to the positive terminal on the power supply. Attach one end of the second wire to the negative terminal on the power supply.
Attach the second lead of the first resistor to the first lead of the second resistor, and twist the wire leads together. Attach the first lead of the first resistor to the free end of the first wire, and twist the lead and the wire together. Attach the second lead of the second resistor to the second wire, and twist the leads together.
Turn on the power supply. Attach the red (positive) multimeter probe to the joint between the first and second resistors. Attach the black (negative) multimeter probe to the second lead on the second resistor. Turn the multimeter to “Volts DC.” The voltage reading should be approximately 6 Volts DC.
TL;DR (Too Long; Didn't Read)
Depending upon the tolerance values of the resistors, the measurement should be within 10 percent of 6 Volts.
If you prefer, you can also reduce voltage by constructing a plug that incorporates a step-down transformer. This is the idea behind the black box power source that is often supplied with computers and other electronic equipment.